I have the feeling that OPAM team is not willing to support OPAM 1.2.2
for a long time. So it doesn't make sense, at least for me, to
include it in Buster.
Yesterday, I've uploaded opam-file-format to NEW. As soon as it gets
accepted, I'll upload latest version of OPAM to Debian/Sid.
It's based on the discussion with upstream at
https://discuss.ocaml.org/t/the-forever-beta-issue/1779/6