> 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.

You could fix versions, and probably should. However willful disregard of prior interfaces encourages developers code to follow suit.

It’s not like Clojure or Common Lisp, where a decades old software still runs, mostly unmodified, the same today, any changes mainly being code written for a different environment or even compiler implementation. This is largely because they take breaking user code way more seriously. Alot of code written in these languages seem to have similar timelessness too. Software can be “done”.