We are breaking our monorepo into 3 kind-of mono repos based on release cadence. A repo for each of two large applications plus a third repo for shared library code. We import the share library code onto the app repos using a git sub module. Has anyone else used git sub-repos to import common code?
We've used submodules pretty extensively in the past but we're working to get rid of as many of them as possible. A friend and former colleague liked to say they were the worst solution... with the exception of all the other solutions. Practically speaking they are probably more confusing to new developers who haven't had a lot of experience with them. We would often run into problems that stemmed from someone making a change in the submoduled repo and then forgetting to update the ref in the parent. Or people would navigate to the submodule to do the work there rather than checking it out separately, and then be confused over the detached HEAD status and check out a branch (not a problem, necessarily, but confusing to some extent). For some of our dependencies we've gotten rid of submodules in favor of hosting internal pypi packages.
Git submodules have a comically bad user interface imho but they work really well so I use them despite this. As with git in general the core functionality is solid. There is also something to be said for making as much of core functionality without resorting to another piece of infrastructure. People can use whatever front end they prefer on their machine but in a pinch the core git package is all you need.