That has never been supported: https://wiki.debian.org/SystemDowngrade
Too many bits of 'advice' on Stack Overflow, etc. claiming it's possible as top Google results.
I'm here to say unequivocally: it does not work, will not work, and will leave the system in an irreversibly broken state. Do not attempt.
But this isn't something that would be 'out of the box'... but that's why we make backups, but I can't remember an dist-upgrade ever significantly bricking my systems.