> Part of the active maintenance is to decide whether to upgrade to a new compiler toolchain version

Of course, but you want to make that as easy as you can. Compatibility is never binary (which is why I hate semantic versioning), but you should strive for the greatest compatibility for the greatest portion of users.

> FWIW, trying to build a 20 or 30 year old C or C++ application in a modern compiler also isn't exactly trivial

I know that well (especially for C++; in C the situation is somewhat different), and the backward compatibility of C++ compilers leaves much to be desired.