[Aissen]

RHEL has gcc-toolset-N (previously devtoolset-N-gcc) for that. It's perfectly fine to only support building a project with, say, the penultimate gcc-toolset. Or ask for a payment for support, which is the norm in this (LTS) space.

[MereInterest]

Oh, absolutely, and I usually push for having users installed a more recent compiler. The problem comes when the compatibility policy is defined in terms of the default compiler provided, because then it requires a larger discussion around that entire policy.