For clarification I want to give you all a simplified plan as far as OUR builds go:
Linux 32bit / EL 6 build env / GCC 4.9 / GTK 2.24 / Minimum glibc 2.12 / Ends November 30th, 2020
Current Linux 64bit (GTK2 Version) / EL 7 build env / GCC 7 or 8 / GTK 2.24 / Minimum glibc 2.17 / Ends June 30th, 2024
Future Linux 64bit GTK3 Version / EL 8 build env / GCC 8+ / GTK 3.22 / Minimum glibc 2.28 / Ends whenever EL 8 does
Long Term Linux 64bit GTK4 Version / Who the hell knows yet...
Let me restate that these combinations and this plan is for production of OUR released binaries. System Packagers and Power Users may use what is appropriate for their system and we doing UXP Development are being very mindful to not unnecessarily bust building potential without cause of which I can see none at this time.. So you can compile a GTK2 build using GCC 5 on a 32bit machine with a glibc version 2.42 if you want. I see no reason to artificially prevent you from doing that.. but of course I don't recommend it.

ALSO I talked to Travis early this morning and he is on board with all this and likely will be taking over builds again by the next release and the unstables now.
BinOC Projects will also likely align with this plan and why not? I came up with it.
THE ONE uncovered point as of yet is Basilisk.. That will require some thinking but expect it to likely follow the plan for the Future Linux GTK3 version eventually.. Whenever it seems like a good time for that to happen. Perhaps sometime next year.