Yes, the opam repository has recently been working on an archival policy to reduce the size of the checkout and hopefully ease pressure on the dependency resolver by pruning 'obviously' wrong choices. However, the heuristic they chose seems to have mainly assumed that the things in the repository are libraries, and used dependencies for tracking usages. For executables like dune, this is obviously the wrong idea, and I think they're still deciding how to proceed while adding back most versions

It is still there according to "opam show dune --all-versions", so no, it has not been removed.

Anyways, what you could do is:

  opam pin add dune 3.7.0
  opam install dune

Alternatively, you can use the tarball directly:

  opam pin add dune https://github.com/ocaml/dune/releases/download/3.7.0/dune-3.7.0.tbz