We've got a old legacy multiplatform app that supported OS X for years. Looked at updating it for Catalina, was going to be a massive amount of work, since Apple also killed XWindows and updated OpenGL in some breaking way. Decided it wasn't worth spending months on it.
Still get a regular trickle of emails from schools complaining about it. I tell them to call Cupertino.
Catalina didn’t kill XQuartz, nor did Big Sur. It still works fine, though it hasn’t been updated in 4 years.
It also did not break OpenGL, only deprecate it. macOS is still stuck on a many-years-old version of OpenGL, but what worked in the past continues to work. Even Apple Silicon Macs will support it.
Still get a regular trickle of emails from schools complaining about it. I tell them to call Cupertino.