I use submodules with jj, and jj saves and restores submodule hashes perfectly. What it doesn't do is manipulate the sub-repository from its parent. You can do that yourself using jj or git of course, which is what I ended up doing using a few scripts. The result ended up being more reliable than using git's submodule commands directly.

They can take all the time in the world to implement submodules as far as I'm concerned. jj's implementation of workspaces removes all of the hairs of git's worktrees. git submodule implementation has even more hairs than worktrees. If the jj developers need time to do as good a job on submodules as they did with workspaces, then I say give it to them.