Yes, the switch to renaming can change behaviour in some corner cases (I say "corner cases" because the expected situation is to have your pyc files be regular files, not symlinks or character devices). Python is certainly not the only application where you can bust /dev/null by specifying it as target location.

Mutating a file in place is a source of unpredictable issues with concurrent access of the file being written to, which is why it was changed to renaming. I think the class of issues which was solved (presumably) is much more important than the use case of compiling to /dev/null.

As for symlinks, I'd naively expect breaking symlinks to be a feature, but YMMV.