Thanks Michael!
This does seem to be related to an interaction between Catalina, Metal drivers, older integrated video cards, and RimWorld's version of Unity. It's crashing as it tries to initialize a rendering surface for the whole screen, which is a low-level graphics init function.
While I'm annoying that Apple released an update that broke the game, I can understand players are more annoyed at not being able to play it.
We can't quick-fix this because it doesn't relate to code we wrote; it comes from code in the Unity engine/MacOS/Metal driver. The only real option seems to be to update Unity wholesale to a new version several years ahead of the old one, but that is a major overhaul that would break everyone's games. I do intend to address this, but I can't promise a quick immediate fix to it.