I'm starting the planning for a new steel road build and I have a choice between a frame with a traditional threaded steerer vs. a more modern threadless. I already have a vintage build that's reserved for Eroica and the intention for the new bike is an alternative to my main carbon road bike for endurance rides and centuries. The groupset is going to be a modern Campy 11 or 12 speed.

I'm leaning towards threadless headset and cockpit setup but I thought I gather opinions.

If you go 1" steerer you have the option of changing your mind. I had my steel forked ti bike built with 1" threadless, I haven't changed it but have been tempted. A Nitto Pearl would look very nice and I love the simplicity if simply unscrewing one bolt, a tap, then any stem height I want and tighten, no torque wrench needed. (No messing with headset adjust either.) Easy to do mid ride.

The change is low on my list but easy. I do have to calculate the stack I want (or have the builder do it), then it is just - hand him the fork, have him cut and thread it and install with a new headset. If I had a good looking Pearl and Stronglight HS in hand, I'd d it tomorrow.

I can never decide until I build the fork. There is something to be said for the ease of adjusting bar height afforded by a threaded steerer. And it's definitely more classic. If it's going to be more of an everyday bike, I think I like threadless better. I'm not sure there is a lot more to it than that.

I like threadless. Details/context would help me (at least): Fork material--steel or carbon? Fork rake style--straight blades or curved? Steerer diameter--1" or 1 1/8"? Depending on the choices for those, that can tip things toward either a more classic look (with sweet modern Campy) or more modern 'hotrod steel' look. Do you have a color or color scheme in mind?

With a very modern drive train, I'd lean towards threadless

I have had a couple of threaded forks fail by cracking through the thread. So far, no failures of threadless forks. And since by now I know fairly well how I like the fit to be I’d be inclined to go for threadless unless there was a particular look I really wanted to achieve.
One reason that might sway me to go threaded is how high the bike is up front. I have a steel Bianchi that would need a ridiculous stack of spacers to get the bar about where I want it if set up threadless. Threaded, no big deal.

Above comments are all valid, especially RiddleOfSteel. My 2¢ is I have several of both and gone back and forth with some, my only hard and fast rule is not on a fine old Italian with Cinelli, Modolo, 3TTT, etc. Now in my 70s my bars are rising and the Italians are not getting ridden.

1 1/8” threadless will give you a ton of stem, fork, headset options. Plus you can customize with funky top caps!

What bars you want to use? That’ll lead to stem clamp diameter, and threaded/threadless will shake out of that. I’d choose 1 1/8” threadless, but that’s me. Especially given Campy 11/12.

New build? For sure threadless. There is just no downside.

Threadless is easy to set up, easy to service headsets, easy to adjust height(you literally just move a spacer from above to below or vice versa), and gives you a stiffer cockpit.
I guess some could argue that they want a looser cockpit where the bars are bending back and forth when climbing, but I am not sure that 'feature' is a widely held opinion. Threadless also opens up tons of stem lengths, stem angles, and countless handlebar shapes.

I continue to not understand why anyone changes their stem height with any sort of frequency. This is a common reason to embrace threaded steerers and quill stems, but I just dont understand why I would change my stem height after its set up with comfort. People give reasons though, so clearly there are valid reasons in some people's eyes.
With threadless, just keep 10mm or even 15mm of spacers above the stem. If you really need to change your stem height, then move the spacers and you are done. If you want more adjustment, then get a different stem angle.
This all assumes the bike you are having built is built for you- so there just isnt a need to adjust the stem height more than a little bit up or down since the bike fits you to start with.

Innicycle.
Threaded fork
Threadless stem

That will be $3.50