I have a Shimano chain for XT , 1x12 CN-8100 and the chain weare indicator enter if I push it with my finger at 0.5%. Is it time to change the chain or should I leave it until it fits freely, without being pressed?

When in doubt, change it. Your cassette will last longer and if you have done it before, it takes less time that following this discussion. Really, it is that quick.

Just make sure you buy from a reputable source.

Replace it.

I would replace it. If you replace it earlier you can get a couple chains on a cassette (sometimes more) and your chainrings will last longer and everything is good. Would agree on making sure you get your chain from an authorized dealer ideally your local bike shop.

I might be a little behind on the modern 1x12 terminology... So, pray tell, what exactly does "chain [...] fits in my chainring if I push it with my finger at 0.5%" refer to?

I apologize! I made the correction in expression. Google translate!

Get out a ruler and measure the length of 24 pins (12 inches of original length). If it measures over 1/16" (1.6 mm) elongation, replace it. Chain checkers are notorious for their inaccuracies. Rulers are NEVER wrong.

That's what monarchists would have you believe.

When I first read this, I thought you wrote "machinists"

Machinists aren't wrong - they're just off by ± 0.001".

Rulers can be wrong if one is not careful.

I use both the ruler and go-no go gauge for checking chains, with a preference for the ruler. First, in case someone hasn't done it before, the chain being measured has to be under tension and can't be slack. Second, it's a good idea to look for tight, sticky, or bent links in the segment being mentioned - if the chain isn't straight, it'll throw off the readings, which can happen easily with a rusty or grubby chain. Third, a grubby or waxed chain can sometimes mask wear by partially filling in the wear areas just enough to make it look like everything's better than it is, even under tension. I wax most of my chains and I try to measure at the end of a cycle, not after a fresh waxing (also keeps me from wasting a wax dip on a bad chain).

But rulers themselves can be incorrectly marked. Reputable Park, Westcott, or other name-brand rulers are probably OK, but I've been astonished by the number of off-brand rulers and tape measures (typically from discount or other stores) where the marks just don't line up with a good known reference scale, sometimes by quite a bit. Several years ago, I purchased a drywall square from a major home improvement chain with US Customary inch marks - it's square, but I was wondering why my cuts were consistently slightly off. I compared each side to my trusty old Lufkin, and discovered it had been printed where the scale was off by about 1/8" in 36". Grumble. I still use it, but it has big markings on it saying DO NOT USE SCALE.

I typically replace a chain at 1/16" wear elongation over 12", or 0.5%. This helps to prevent premature wear or failure of the chainrings, freewheels, cassettes, or sprockets. I have more than one freewheel with more than 10,000 miles on it that still shifts like new, and I can get over 10,000 miles out of an aluminum chainring (but those do eventually wear). If I treat a chain well, wax every 500 miles, and flush & dry it after wet weather rides, I can get from 3,500 to over 5,000 miles out of it before hitting 0.5% wear.

If you have to ask...

It's preventive maintenance to change the chain when it gets to a certain wear point. Whether at the 0.5 or 0.75 mark.

Preventative maintenance can be more expensive than just waiting till there is a problem. Essentially you have to consider if the cassette or chainring will be more of a cost or problem for you if you were to run the chain till your drivetrain works no more. But for some of the cassettes with lots of sprockets that are more expensive than two of your chains, it probably is worth it.

Only if you race in competitions, can I imagine that you absolutely have to do the preventative maintenance things that are claimed here by many. Perhaps if bicycles also flew, it would be a must.

Most probably like tinkering with their bikes. So if that's you. Change the chain, cassette, rings.... whatever makes you feel happy.

FYI, I have a bike with a 11 speed rear. I change mine when it gets beyond the 0.5 mark. Not because I think I'm saving money. After more than 8000 miles with nothing to do to my bike except changing tires, I'd welcome the tinkering myself. Though currently the chain has almost 8,000 miles. But it isn't more than 100.5% elongated yet.

So a dilemma. To tinker or not to tinker. That is the question!

the ruler he or she uses will be metric, i'd guess... a swiss or germanic last name... "Google Translate" mentioned, etc.
12.7mm bicycle chain link pitch, 609.6mm in two feet, ... about 3mm in 609.6mm length equals .5% wear...
two feet, 610mm, on a chain, is an easier read on a ruler

i check chains by a push-pull of three links.... and side motion too.
there is a technique using the chain, and pulling it outward from the chainring too... that can also show chain ring wear, and the amount of pull out can vary by tooth count on the ring involved...so can be a bit misleading to most people.
the chain wear indicator stick takes away the doubt... totally perfectly accurate or not.

considering the recent Prices of a new "12 speed" ANYTHING... get the new chain now.

... then you want to learn.

sentence completed.

I see this a lot. It may be true that rulers are more accurate, but I've always struggled to read the small increments accurately. I've come to the conclusion that perhaps it's a form of dyslexia; I'm fine with writing, but I find numbers and rulers difficult, particularly numbers in small point sizes, which appear to jump around. A chain gauge, on the other hand, is dead straightforward. I think the trick is to know exactly what it read when the chain was new. Now that I'm using the same chain on both my road bike and mountain bike, that's quite easy, and I can clearly see any deviation from that.

I don't believe that such a simple thing as a chain-checker from a reputable mfr (Park Tool) would be inaccurate in its measurements.
And rulers can be difficult to use and read for some, especially if putting the chain in tension in order to be accurate.

I went to college in the slide rule days. If you couldn't interpolate between the hash marks on your slide rule, you were going to have significant difficulty in engineering school. Absent a specific optical disability, learning to do this is a skill that can be developed.

I grew up with a slide rule as well - in fact I still have it. I can tell you grew up in those days too by the fact that you seem to think dyslexia is something that can be overcome by developing a skill; it can't.

I'm not saying I couldn't use a ruler, but I'd really struggle and I therefore can't see the point. It would definitely not be accurate for me.