Through Abhilash's diligent work, we've finally brought all Mailman 3 development under one group on GitLab.
This includes the Core, Postorius, HyperKitty, the client library, the bundler, Postorius-standalone, documentation, and even the soon-to-be deployed new website.
Please remember that Launchpad is still used for Mailman 2.1, and there are no plans to move any of the project artifacts for that series. All Mailman 3 issues, feature requests, merge requests, etc. for all other related projects should be filed against the appropriate project on GitLab.
While we're at it, I'll also point out that Abhilash has pushed out a rtfd site for the Mailman Suite, with pointers to the various sub-projects: