Apps that use OpenGL are still available on Big Sur (macOS 11.0) even on any upcoming Apple Silicon Macs.
macOS 11.1 or later could drop support entirely, of course.
Is your worry that a current 4coder binary won't work on macOS Big Sur?
It should run fine if it currently runs on Catalina (at least with respect to OpenGL vs. Metal).
Or are you concerned about writing OpenGL code as part of your own "extension" (or binding) C code?
Sorry, I've not used 4coder, so I'm not sure if it's possible to easily hook into 4coder's OpenGL context, but if so it should work fine as long as 4coder is using OpenGL, and you'll have to transition your code to Metal if a future 4coder uses Metal.
1. Allen updates 4coder to support Metal (if not already), and you can update your extension code to use Metal instead of OpenGL.
2. Allen doesn't add Metal support to 4coder and you can run it until Apple no longer supports OpenGL in a future macOS 11.1 or later. You could always not upgrade, older versions are supported for at least a few years.
3. There's always the newly announced *free* VMWare Fusion Player that you should be able to use to run apps/games that use OpenGL and not Metal well into the future. (though not a guarantee, but more of a likely case)
4. It's also possible that Apple's Rosetta (2nd gen) that will allow x86 code to run on Apple Silicon and currently supports apps/games that use OpenGL.
* Having said all that it is recommended if 4coder is going to be supported beyond 2020 that it should switch its rendering layer to use Metal since there could be bugs that are either never fixed, or take a while before a fix is released (e.g. Rosetta's OpenGL breaks for some reason).