Hmm, I've done some googling and it looks like Valve has a 64 bit version of Steam for macOS already. You may need to delete Steam.app and re-install to get it.

Hmm, I've done some googling and it looks like Valve has a 64 bit version of Steam for macOS already. You may need to delete Steam.app and re-install to get it. So you will still be able to use Steam itself on Catalina+, but will need to wait for 32bit only games to have a 64 bit version made available (or for someone to come up with some kind of compatibility tool)