True, but they didn't remove OpenGL, they simply deprecated it (e.g. don't expect any updates to it, new tooling will not be built around it, etc). That shouldn't affect legacy apps.
Yes, and deprecation doesn’t mean a lot on the Mac. Apple often deprecates stuff and still leaves it in. They remove it only when there’s something to be gained.
(eg. linking with the system-provided OpenSSL has been deprecated for years, but AFAIK they still ship it.)
Apple can get away with that on iOS, but they're a lot more conservative with macOS.
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.
They said in the Platform State of the Union that Mojave will be the last macOS that runs 32bit apps, so QuickTime.framework and your app are running out of time!
True — 10.12 is my recollection as well — but I’ve been bitten so many times by compiling under a new SDK, especially with an older build target, that I do that as a matter of course anyway.
I find it hard to fathom that people think a huge software company like Apple doesn’t have awareness of the impact of its changes or people responsible for compatibility.
