BGL is deprecated and will not work on Metal devices. Although the
inital plan was to remove it in Blender 4.0, We don't see any harm
to still have it in the code-base until OpenGL itself is deprecated.
Add-on developers are warned when using the BGL module that the
add-on/script will not work on all platforms.
There are still some limitations inside the GPU module that needs
a more friendly API. This API isn't clear at this time.
Jeroen Bakker