> Much more important is reach and standover height, since those can’t easily be changed.

I try, buy and build quite a lot of bikes (6 this year). Reach has never been very helpful for me. Seat tube angles differ quite a lot and the position of the bottom bracket on the x axis does not affect me that much. Effective top tube and stack is what I look for.

Being a long legged person, standover has never been an issue. I can straddle most any bike. Stack is important because that gives me a hint if I can get the saddle and handlebars on the same height without crazy stem solutions. I ride bikes from the horizontal top tube era that favor the "fistful of seatpost" rule, so C-T seat tube length is also very important to me (for purely aesthetic reasons.)

Seat tube length only really makes sense for sizing for classic flat top-tube road bikes. It definitely doesn't make sense for aggressively sloped top tubes on carbon MTB and road bikes.

Most vendors these days seem to do Small/Medium/Large (etc) sizing, rather than seat tube measurement. It isn't portable across bikes, but neither would seat-tube length be.

I disagree that standover height has much relationship to fit. There is some practical aspect, especially for newer riders. But you can have a very high standover with a high bottom bracket with similar fit to a lower standover with lower bottom bracket.

My thoughts exactly. Being on the shorter side what matters is the stand over height - too high and it greatly limits on/off/standing manoeuvrability and agility. I was surprised to learn that the smallest adult bikes tend to have a stand over height of 68-70cm. For some bikes, it’s even higher. For the lower tail of the adult height distribution, this is too tall to use comfortably.

This is a sad topic for me. It just doesn't exist a premium model all mountain bike, that I have even 1mm of clearance between me and the top tube. Even when standing on my toes.

Look into bikes with 650b wheels for this.

Pay attention to BB height, specifically. That'll dictate ground clearance and comfortable saddle height. 650B wheels are often fitted with bigger tires, and can have the same effective diameter as 700C wheels with smaller tires (depending on specific tire size).

This is correct. I was in a hurry and didn't explain at length as i should have.

There is a slow movement to using stack and reach for frame sizing, which are the two most critical measurements for bike fit.