I am not sure whether it affects all versions of macOS, yet I do know that it crashes on these versions of the Operating System. It occurs on the latest version of the software.

If you’re using Prism Launcher (or any MultiMC based launcher) environment variables settings are available in the “settings” tab in instance settings

How would I add it into Environment Variables?

It is crashing because Legacy4J embeds libsdl4j which embeds SDL2 shared libraries for windows and linux only. I got it to work by showing SDL2 from homebrew via DYLD_LIBRARY_PATH environment variable.

Steps:

1. Install homebrew (brew.sh)
2. Install SDL2 from homebrew (brew install sdl2)
3. Set additional environment variable through your launcher (Prism Launcher can do it, I don't know about others) like this: DYLD_LIBRARY_PATH=/opt/homebrew/Cellar/sdl2/2.30.2/lib/
4. Try to run the game

Fixed in version 1.6.4, thanks for reporting.