It's probably good that it's possible to opt-out of being archived but what annoys me is a useful domain changing hands and the new owners deciding to wipe its history.

Surely that's not possible?? And there is no way to find the old archive, even under an alias?

It's not wiped (they don't delete anything) but if robots.txt forbids indexing then access to old archives is blocked. It can be quite aggravating, but at least the information is not lost---if robots.txt or archive.org's policy changes, it can be retrieved again.