I'm on MacOS Catalina, which unfortunately only runs 64-bit games.  Right now, it appears that the only way to tell if a game is 64-bit is to download it and then try to run it. This is very inconvenient, and pretty much rules out buying games. Would it be feasible for this information to be on games' pages? I would also greatly appreciate if the Itch app were able to sort my library to only show 64-bit games.

Perhaps you could use Steam's interface as a model. On the Steam store, every 32-bit Mac game displays this notice:

"Notice: This product is not compatible with macOS 10.15 Catalina. Click here for more information."

Additionally, Steam's library has a very convenient button that only shows my games that have 64-bit Mac support.

I really really love Itch, so it's saddening that Apple's banishment of 32-bit games has made Itch nigh-unusable for me. Any solution would be greatly appreciated!

That's a good idea. In the mean time, you could try and pester creators of commercial games to add free demos, which would help.

Generally having minimum/maximum OS options would be useful, especially for said Mac OS games, but also Android games.

Same. Yes please. So tired of buying games just to find out I can't play them AFTER I've already paid for them.