Yeah, we had that discussion quite often over the last few years. I mean to remember that the extension is covered by a patent and thus by licensing fees and the ARB does not want to put something into core, that forces vendors to pay a fee to a third party.
On the other hand it is so widely supported, that it doesn’t matter whether it is in core or not, it will always be there.
Also, it only changes the very final output. So if you enable it, you get nicer graphics, that’s it. It does not change how you program your renderer. That means, in the rare case that it is not supported, your code does not need to change, at all.
So, even if it were missing on some hardware, it wouldn’t be an issue, at all.