Opam 1.2.0 will be actively deprecated in favour of opam 1.2.2, which now becomes the only supported stable release.
Why deprecate opam 1.2.0
OPAM 1.2.0 was released in October 2014, and saw rapid uptake from the community. We did some rapid bugfixing to solve common problems, and OPAM 1.2.2 was released in April 2015. Since then, 1.2.2 has been a very solid release and has been the stable version in use to date.
Unfortunately, part of the bugfixes in the 1.2.2 series resulted in an
file format that is not fully backwards compatible with the 1.2.0 syntax, and
the net effect is that users of 1.2.0 now see a broken package repository. Our
CI tests for new packages regularly fail on 1.2.0, even if they succeed on 1.2.2
As we prepare the plan for 1.2.2 -> 2.0 migration, it is clear that we need a "one-in one-out" policy on releases in order to preserve the overall health of the package repository -- maintaining three separate releases and formats of the repository is not practical. Therefore the 1.2.0 release needs to be actively deprecated, and we could use some help from the community to make this happen.
Who is still using opam 1.2.0?
I found that the Debian Jessie (stable) release includes 1.2.0, and this is probably the last major distribution including it. The Debian Stretch is due to become the stable release on the 17th June 2017, and so at that point there will hopefully be no distributions actively sending opam 1.2.0 out.
How do we deprecate it?
The format changes, although small, would cause errors on 1.2.0 users with the main repository. To avoid those, as was done for 1.1.0, we are going to redirect users of 1.2.0 to a frozen mirror of the repository, making new package updates unavailable to them.
If there are any remaining users of opam 1.2.0, particularly industrial ones, please reach out (e.g. on Github). By performing an active deprecation of an older release, we hope we can focus our efforts on ensuring the opam users have a good out-of-the-box experience with opam 1.2.2 and the forthcoming opam 2.0.
Please also see the discussion thread regarding the deprecation on the OCaml Discourse forums.