Me, CS5/6, Quark and all other 32bit, although most of mine are 64bit Apps. I can run 10.5.8, 10.6.8, 10.9.5, 10.12.6, 10.13.x, 10.14.x, 10.15.x on my MacPro 3,1 they all run great. I'll go play every so often and have found that 10.12.6 is just fine. But only certain apps work well as you go up. AND I am NOT moving into a "subscription pay" app. The new computer con.
As the newer OSs come out, just like the hardware, there are a ton of BenchTesters/Orgs that publish their results online. And not too long ago I read a compliation article that said the newer OSs will run on and found that they run better on the older hardware. Apple is just locking those machine out so you have to buy a new one. With Apple there is no "Planned Obsolescence" just "Forced Obsolescence".