+1 for subrepo.

My concern with subrepo is that I have no idea who is developing it and how much resources they have. I'd hate to learn to use a tool central to my development workflow and then have it discontinued.

Is git-subrepo good enough that it's worth the risk of using something not built into git (and the hassle of installing something extra)?

For a minute I confused this with another external tool: git-filter-repo [0]. It's recommended by the official manual as replacement for git-filter-branch [1].

[0] https://github.com/newren/git-filter-repo/

[1] https://git-scm.com/docs/git-filter-branch