For myself I haven't found any yet, but I just got back into riding. As far as others go? I am not full enough of myself that I worry about what others do.

I have very seldom paid over $1k for a bike. I did splurge on a Bike Friday New World Tourist but that's about it.

My "no, thank you" list:

Non-ferrous frames. Aluminum, titanium, carbon fiber, unobtanium need not apply - I'm just not interested.
Four-arm cranks. Still just plain fugly to my eyes.
E-shifting. Just another thing to go wrong on a ride.
Down tube shifters. Nothing wrong with them, and they make recabllng much less complicated, but I'm a bunch less flexible than I used to be.
Toe clips and straps. Look cool and I rode a pair across the USA (granted, that was 45 years ago), but again, I'm not as flexible as I used to be. Plus, SPDs are a lot more comfortable.
Single-bolt seat posts. I mean, I will if I have to, but they are invariably impossible to set just exactly where I want. I'd much rather wrestle with a traditional Campy 2-bolt (and that really isn't that hard with a 10mm ratcheting wrench, swivel nice but not essential). Nitto Frogs would be ideal if they came in more diameters.
Non-aero brake levers. Because of my decreasing flexibility, I have to be able to brake from the hoods. In my experience, aero levers have better mechanical advantage than non-aero levers, and I want my brakes to work, especially since I'm a big boy who generates a lot momentum on descents. (For the same reason I generally prefer dual pivot brakes, but the Suntour single-pivots I have on one bike do the job just fine - and yes, that bike has aero levers.)
Tubulars. I went away from them in early 1980s, tried them again for Eroica CA about ten years ago. For me, they are no longer worth the hassle, especially since my usually sluggish pace does not give much if any of their advantages, especially since I ride Conti 5000s and rode the 4000s before them.
E-bikes - for now and for the foreseeable future. But as I start my still-early-stage slide into decrepitude, I have to take a "never say never" approach. But the motorcycle-replacement urban terror machines are a non-starter.

My largely but not completely heretical "yes, please" list:

First, the non-heretical ones - Steel frames, preferably lugged or fillet brazed, but nice TIG welds will do in a pinch.
San Marco Rolls saddles, especially the Titanios. They just work for my butt, so I've stopped looking for others. I do have an old Brooks Pro and a NIB B17 in reserve, just in case.

Now for the more sinful stuff - Campy 10sp triple drivetrains. Look good, work well.
SPDs. I have 'em on almost all my bikes and they work great for me.
Tall Nitto stems. I like the Tallux that Rivendell sells (Technomic height, nicer finish).

Carbon bike, I’m just too clumsy, I would be dropping it and then worrying if it was going to blow up on me. Just not worth the stress to me.

Also hydraulic disc brakes, hate the look and seem like unecessary bother considering my rim brakes all work, even the rod brakes!

Seriously? Greg LeMond and I were both using an Avocet bike computer in 1984, 41 years ago. If that isn't vintage then I don't know what is. Seriously though, I only have one bike with a bike computer (an '86 Avocet that's had a dead battery for the last 5 years or so) and the only computer I use is the iPhone in my jersey pocket.

I have no lines in the sand. One man's (or woman's) modern is another man/woman's vintage, and a few years from now Di2 will be considered vintage. I even have a 25 year old mountain bike with shock absorbers. Anybody want to tell me that's not vintage?

I’ll try most things once (not including double black Diamond MTB runs).

Tubulars: Rode them once and they were amazing, but the setup was a royal pain and after peeling one off in a hard corner, never again.

Toe clips: I hardly even ride clipless anymore. I won’t get hung up in those anymore.

Headset routed anything: Having witnessed their folly in the bike shop, this was a solution to a non-existent problem and caused more problems.

Other than that, I have preferences and will ride just about anything - even a Schwinn Varsity!

There are no lines I'm not willing to cross. However, simply because I keep investing in older bikes and equipment because I like it and frankly because it's become habitual, I have yet to try tubeless tires, disc brakes, or electronic shifting even though they are all ubiquitous now.

-Gregory

I can't get behind tubeless tires, personally. I also don't want to use an electronic shifter; in my car, both of these are fine, on my bicycle no thanks.
These are not hard lines in the sand but I don't see myself crossing them anytime in the future. I'm not a luddite, but I grew up in the Boy Scouts and no one can tell me that with tubeless tires I simply won't get flats. The only way that happens is if I don't ride. To build upon that, no one will ever convince me that a flat won't happen at the least convenient time and place. I really want the ability to easily repair a flat and get on with my ride.
The same goes for electronic shifting. I don't feel like carrying batteries and I'm not good at changing batteries or charging anything besides my phone, my computer, and my hearing aids. At least once a year I find myself out on my bike with my Cateye Padrone (the most advanced bike computer I think I'll ever get) and either the head unit battery dies or the sensor battery dies. They are easy changes, but it always seems to happen out on the road and I always overlook the periodicity of that maintenance since there's no battery gauge on the device. So there's no way I'm going to allow myself to get caught out in BFE (or north Philly for that matter) without the ability to shift and fix my own flats.
Phil

Bib shorts...

Pneumatic tires! Good old solid rubber never flats, much more reliable... Oh yes, Chains! They're complicated, have to be lubed constantly, and they wear out regularly.
Brent

This explains all of the penny farthings I saw in your garage!

I appreciate both vintage bikes and modern bikes and I don't have 'lines' exactly, but there are things I'm not personally interested in using:

• Too much technology on the handlebars - a bike computer should have just a few buttons and simple data fields during the ride. I ride bikes to get away from watching screens.
• Tight leather, leather bondage straps and metal cages - specifically saddles and pedal clips. They're sexy to look at, but painful to use and not in a good way.
• Tubular tires - I roll my eyes when anyone complains about the minor annoyances of tubeless, because these are nothing compared to the sticky horror from the dark ages of tire technology.
• E-bikes where the motor is more powerful than the rider - I don't care how carefully tuned the assist is to feel like pedaling, if the machine is clearly more powerful than me then I'm operating a machine, not riding a bicycle anymore.
• Brakes that use corrosive fluid that breaks down plastic, paint, and skin - I am on team mineral oil.

I avoid TIG welded aluminum frame. When I started cycling during Covid, I had a hand-me-down TIG'ed aluminum bike and the ride was horrendously bad. Which I realized when I switched to a steel Lemond bike. Later on I bought another TIG welded aluminum bike to harvest the groupset and wheels, and likewise the ride was horribly stiff on this bike. I couldn't wait to strip the parts off and throw the frame in the recycling bin. So no more pre-hydroformed TIG welded aluminum frames for me.
The other thing I dislike, is threadless stems. I insist on quill stems. I actually have two carbon bikes with quill stems. Carbon OK. Threadless not OK.

My philosophy used to be to keep a bike as simple, universal, and on-roadside-repairable as possible, so it used to be clipless pedals, then STI/Ergo Shifters, then carbon frames, full suspension mountainbikes, then aero and integrated framesets. Currently, it would be electronic/wireless shifting. I've broken all the older ones sooner or later, of course; so it is hard to rule anything out. I can say i will keep on drawing the line on thigh high socks.

My sand shifts with the tide, and storms. The above post regarding high socks; I have embraced knee high socks with knicker type shorts for chillier rides. Thigh high,not socks, I have leg warmers.

My line in the sand is, No hydraulics, electric or electronic....But that may change as I go through my cycling journey.
...mid

Anything unnecessarily heavy, ugly or expensive.

I drew that line in the sand, then the tide came in. I was NOT going to modernize the lovely Italian race bike I found here on BF. Not going to:
Buy a new wheelset,
Add brifters,
Install a modern low geared drivetrain,
Spread the rear triangle,
Shimano anything.

Not sure if that qualifies as C&V versus post-C&V, but a good point regardless. I have one single-bolt, and I am getting rid of it! My two choices are saddle dead flat and me sliding off the nose, or the saddle looking like an airplane taking off.

Very simple, here are the mandatory things for me:
-Look PP pedals
-Easton or Look or Ritchey carbon seatposts if not Dura Ace 7410 seatpost
-Dura Ace 7800 groupsets
-FSA carbon or Dura Ace 7700 or Dura Ace 7800 cranks
-Continental Grand Prix 4 seasons tires in 700*23 or 700*25
-Selle Italia or Fizik saddle
-3T handle bar and stems on the aluminium road bikes and ITM bars and stems on the steel road bikes
-Zipp Carbon or Mavic Cosmic Carbon or Mavic Cosmic Pro wheels

That was my line too. Then I looked in the mirror, got on the scale, and revisited my bank account. I had to move that line so I could go ride my bike.

My wallet. Where ever it lies, therein is my line...which usually means anything new and pricey (I am a big fan of Vittoria Zaffiros, for example...)

Of course, all of the little used CV purchases probably could have been a nice $7000 CF bike...

Same, except the other way around. I rode steel bikes for 35 years and then bought my first welded aluminum frame. That was that. Comparing bikes with the same wheelbase, my aluminum bikes are indistinguishable from my steel bikes in vertical rigidity and superior in torsional and lateral rigidity. (Here's a great article on the topic.)

Still have my last steel bike, a Reynolds 853 Schwinn Peloton, in the basement, but I haven't ridden it in at least 10 years. So I guess that's where I draw my line, come to think of it.

I'm 70 and started out riding/racing on a 5 speed friction downtube shifter with sewups a long time ago.
My line in the sand is what I can afford, want and justify to my cfo.
I like modern technology and currently ride/race a Tarmac SL8 expert with Ultegra Di2 12 speed with Elite 50mm carbon wheels. I built it and love it...one sweet ride.
I do miss riding my old Colnago, SL tubing but even at a 48cm frame I could rub the front der. when sprinting...it sits proudly on my shop wall with a crumpled top tube from a crash at the nat crit a long time ago.

Many of my lines are driven by financial limitations.
More of a preference than lines.
No AL frame bikes
No electronic assist anything
Don't see the benefit of tubeless - How is it easier than tubular?
No sealants due to the low frequency of riding a specific bike. Have over 10 available.
Campagnolo first, Suntour Superbe Pro second, Shimano if I have to or given too me.
Indexing on a road bike was crossed with a 91 Montello equiped with 8spd DA (7400) with down tube shifters. "Brifters" were avoided due to cost and the unlikelyhood of rebuild. Then moved to affordable Ergo's with DA block and eventually 100% Campy - Italian bike, Italian drive train.

Happy crossings:

1. Chain waxing
2. Erogo's
3. Clipless - like KEO's and the one pair of Speedplay.
4. One computer with sensors and mounts on every road bike
5. Going back to tubulars
6. Giving in to using triple crank