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Apparently Gerrit supports this with topics: https://gerrit-review.googlesource.com/Documentation/cross-r...

It is very easy to build this yourself. Just use your dependency manager of choice that has git support and create a config of versions of the repos and lock file that. Whenever you have a change across multiple services, simply change that config in one go, so that every commit of that config describes a deployable state of the whole project.

I might be misunderstanding but that seems off-topic, not related to the idea of "multi-repository pull requests"

At a company I used to be at they used GitHub Enterprise and some repos definitely seemed to have linked repos or linked commits (I don't remember exactly but there was some way of linking for repos that depended on each other).