Ahh so stacked diffs? https://docs.gitlab.com/user/project/merge_requests/stacked_...

Yes, except with a proper UI. Also while you could do this on GitHub before in the same way that that Gitlab feature works, it didn't work cross-repo so in practice it wasn't an option for most open source code.

I can't remember if Gitlab has the same limitations but I do remember trying to use Gitlab's stacked diffs and finding them to not work very well. Can't remember why tbh.