If you installed it using Homebrew, the file containing the library should exist within "freeglut" under "opt/homebrew". You can also drag and drop the folder with GLUT code into Xcode and select the option with "copy" instead of "reference," then make use of the headers.

Not retired, "Deprecated". OpenGL 1.41 is still compatable with modern MacOs, and even has a dedicated driver on M1 computers to run ontop of Metal. Newer versions of OpenGL will never be supported, however.

Could be. I wouldnt blame Apple though since some AMD OpenGL drivers do the same thing