I'm ok with "removing" this "guarantee" (although it seems too late to apply this specific change to 3.7 now).

While Cython users do ask for a way to build cross-version binaries from time to time, it's neither supported nor planned, and the underlying problem is IMHO solved better via tooling like the manylinux wheel build.

I also generally consider it good to let user code take advantage of new features in new CPython releases, even if it means that they need to retranslate their Cython code from time to time. The 1.5 years release cycle of CPython should exceed that of most "still in maintenance" extension modules out there, and it's long enough to not impose a real burden on their maintainers.