This is irrelevant over the long run because the environment changes even if nothing else does. A compiler from the 1980's still produces identical output given the original source code if you can run it. Some form of virtualization might be in order, but the environment is still changing while the deterministic subset shrinks.

Having faith that determinism will last forever is foolish. You have to upgrade at some point, and you will run into problems. New bugs, incompatibilities, workflow changes, whatever the case will make the determinism property moot.

Many compilers aren't deterministic. That's why the effort to make Linux distros have reproducible builds took so long and so much effort.

The reason is, it's often more work to be deterministic than not deterministic, so compilers don't do it. For example, they may compile functions in parallel and append them to the output in the order they complete.