Luckily Steam tells us which games will be incompatible with Catalina. For me it's a big part of my library and some are even recent (-5 years). I doubt these game will ever get updates or Apple will provide a compatiblity layer ala rosetta stone like they did for the PPC to X86 switch. A lot of old games are already lost to me because of this.
I have not looked into this, but if it's a flag you might be able to change it yourself in the info.plist file. Unless code signing get in the way.
It's an issue in Steam database, not in the games. It shows the "this game is not compatible" message, but the games can still run.
Absolutely incredibly how much of my Steam library disappeared overnight. I realize dual-booting Windows or an old OSX release is an option; but not so easy with the minuscule MacBook flash storage.
If you want to go the dual boot way, target mode or an external disk to boot off might be viable.
I dual-boot my Mac mini (different Mojave and Catalina setups) off of a USB-C external enclosure with a 1tb m2 SSD. Boot time is a bit slower off the external drive, but it's workable.
Some of the games that Steam says aren't compatible are still compatible.
Another reason for me to not upgrade, despite the notification nags me every day
It's quite easy to get System Preferences to ignore the update [0]. Ignoring Catalina isn't a long-term solution, but is pretty much a necessity for many right now.