(eg. linking with the system-provided OpenSSL has been deprecated for years, but AFAIK they still ship it.)
To expand on your example, I maintain a legacy app that is stuck in 32-bit land because it relies on the QuickTime framework. QuickTime has been deprecated for seven years, and the transition to 64-bit has been in progress for over a decade, and yet my legacy app runs just fine even under the Mojave beta. There are multiple time bombs lurking in that app, and one of these days I'm going to have to rewrite it from the ground up, but I've been astonished at how long it has lasted.
Apple knows it would be bad karma to make a large number of legacy apps and games suddenly break on the Mac. They're not idiots; they have a perfectly good idea of the scale of mutiny that would ensue. So I'll eat my hat if OpenGL doesn't continue to work for at least the better part of the next decade.
It has entirely been removed from the latest (since 10.12 IIRC?) SDKs so now you have to keep older SDKs just to build your app.