OpenGL Stenciling, separating ref from value written?
In the answer to this question, a vendor specific extension GL_REPLACE_VALUE_AMD is able to do exactly what I'm struggling to do in OpenGL, but I'm worried it will limit what computers and platforms I want my program to run on, and I've had no luck researching where it would not be available.
My goal is for the program to run on any computer that supports OpenGL 2.0, without any functional differences between them. Should I compile a program that uses this extension, what computers/platforms in this set would no longer be able to run the program without problems, if any?
The fact that it's a vendor extension should be an immediate clue that there's a good chance that you'd be limiting yourself to that vendor's hardware. It's not a 100% guarantee; NV_texture_barrier has been implemented for years on pretty much anything that can run GL 3.3 or better.
Further research indicates that the publication date for that extension is from 2012. That suggests that the extension would likely be implemented by more recent, GL 4.x-capable hardware.
If you want more accurate information, there are databases of extension usage that give a clearer picture. From this, we see that the extension is only implemented on AMD hardware. While it is available on AMD's GL 3.x-class hardware, it is not available on any of AMD's 2.x class hardware.
So if your goal is to support GL 2.0 (why not 2.1?) as a maximum, then you can't use that extension.