They sure can, but it seems they simply did not want to.

Maybe - likely we’ll trade-off the added build/test/storage cost of maintaining each variant - so you might not see amd64v4, but possibly amd64v5 depending on how impactful they turn out to be.

The same will apply to different arm64 or riscv64 variants.

Probably not v4 unless AVX512 becomes more ubiquitous than it looks like it will. But yeah, I don't expect this to be the only variant ever.

Yes sure

apt (3.1.7) unstable; urgency=medium . [ Julian Andres Klode ] * test-history: Adjust for as-installed testing . [ Simon Johnsson ] * Add history undo, redo, and rollback features

If you're using btrfs, you do get that feature: https://moritzmolch.com/blog/2506.html

That fundamentally requires a snapshot-capable filesystem, so you need to use a distro designed around such.