I understand how sizing on road/cross/gravel bikes works, but when it comes to mountain bikes of any era, I do not. It seems like the larger frames tend to be smaller than larger frames for other types of bikes based on what I can find for sale. It's relatively easy to find old 58-62cm road/cross frames, but I don't feel like it's at all easy to find old 24in mountain frames, plus it seems from looking at BikePedia that few frames went beyond about 21 or 22 inches, so I really feel like I'm missing some important piece of information here. Did no tall guys buy mountain bikes in the 1990s? Please help me understand this. Also, if it helps I am almost 6-1.

At 6-1 you almost for sure dont need a 22" frame with level top tube.
I am 6'5 and have a 22" frame with a very slight sloped top tube and its plenty big.

Mountain bikes should have more crotch clearance than road bikes. Easier to dismount during a short climb you cant complete or anything else where you need to get a foot down orndismount quickly.
This smaller frame is offset with s top tube which will be long for the frane size when compared to level tube road bikes.

Also, mountain bikes have a higher bottom bracket. This means you are starting out higher up vs a road bike.
Seat posts are now 350-400mm for a reason- mountain bike sizing means more exposed seatpost compared to traditional road bike geometry.

Just to echo what was said above....

MTBs of any era have lower top tubes and thus shorter seat tubes than road bikes. So someone who rides a 52cm (~20.5") road bike might end up on a 17" MTB.

At 6'-1" I think it unlikely you would want anything bigger than 21" and some people your height would ride even smaller, depending on the particular frame and their body proportions.

FWIW, seat tube length has never been a very good way to size mtbs (though it took a few decades for people to catch on to this). Effective top tube length (or reach/stack, if you know it) is a much better indicator of frame fit. I have ended up on everything from 16" - 19" mtb frames for proper fit and handling.

My road bike is 54-55 cm.

On my 29er, I ride a 17". On my old 90s MTB, I rode a 17.5".

As a rule, you size down from a road bike when buying an MTB.

My steel 1991 Fisher Paragon is a 19 and I'm 6-1, 32 inseam. I now ride a Kona Hei Hei size Large for comparison. I rode a 58 Bianchi Virata at the same time.