I've never been able to successfully create a GL context > version 2.1, or invoke the GLSL compiler.
As a sidenote, I've very much enjoyed your blog, and developed a similar handle system as yours around the same time. Mine uses 32 bits though - 15 for index, 1 for misc stuff, 8 for random key, and 8 for object type :^)
Recent versions of macOS will provide either an OpenGL 2.1 context or OpenGL 4.1 context, depending on how you request the context. You have to request a 3.2+ core profile, and not use X11 or the glX* functions.
From macOS 10.7 to 10.9, you'd get an OpenGL 3.2 context. As OpenGL 4.1 is backward compatible to OpenGL 3.2, it's fine that the same code gets OpenGL 4.1 now.
Basically, macOS will provide an "old" API to programs that need it, which is fixed at 2.1, and a "modern" API to programs that know how to ask for it, which has settled at 4.1 and is unlikely to change.
OpenGL 4.1 is harmonised with OpenGL ES 2.0. Almost the same rendering model, features, extensions, etc. On iOS, iPadOS etc you can use OpenGL ES 2.0, and no version of OpenGL (non-ES), so my guess is that's why macOS settled on OpenGL 4.1. Both platforms offer the same OpenGL rendering features, but through slightly different APIs.
But if you request 4.1 over GLX (which uses X11/Xorg/XQuartz), the X11 code only supports OpenGL 2.1. For example, if you're porting some Linux code or other GLX examples over.
Unfortunately, the GLX limitation is probably just due to the Xorg-based XQuartz being open source but only minimally maintained since before OpenGL 3.2 was added to macOS. XQuartz uses Xorg and Mesa, which have all the bindings for 4.1, but some of them are not quite wired up.