With some motivation you could port git-bug to another VCS without too much problem. You would need to implement those interfaces [1]. The one you care about especially is RepoData, which mainly imply you can store a DAG, have references and push/pull. I believe other VCS (say mercurial) have similar concepts.

Or you could just as well plug a generic database there.

[1]: https://github.com/git-bug/git-bug/blob/master/repository/re...