Can I use Steam in macOS Catalina or am I stuffed?
Steam will run fine in macOS Catalina. 

However, what you may find is that it doesn’t launch initially. If this is the case, totally remove it from your system and then reinstall it from Steam’s website after you’re on macOS Catalina. This will ensure that the correct (i.e. 64-bit) version of Steam is installed for macOS.

IMPORTANT

The fact that Steam itself runs fine does not guarantee that all of the games in your library will run fine. So you will need to do your own due diligence to ensure your favourite games are compatible (i.e. they will need to be 64-bit versions to run on MacOS Catalina).