Didn’t know Debian can do that:
I am aware of snapshot.debian.org. It’s cool. Was previously implemented in the build script
Downgrade using snapshot.debian.org isn’t a “real downgrade”. One reason are Debian maintainer scripts. These are really problematic since the work these do is mostly functional and not declarative. Therefore not easily reverted/downgraded.
Most other distributions aren’t much/any better. Therefore APT warns against downgrade because it could leave the system in a broken state. Real downgrades may only be possible with stateless distributions.