Not to mention the default main branch of a git repo is master
We won't change that until git does. Of course you can customize your default branch.
Unlike the 'master'/'maintainer' role, the main branch does not directly correspond to a person.
A master/slave DB isn't a person either and look where that ended up.
Hg sensibly calls the default branch `default`.
Git sensibly calls the master branch `master`.
That's funny, because it's pretty common to set a 'dev' branch to default and reserve the master branch for production code.
Which no doubt comes from the term "master copy", which has a long established, and quite precise meaning. Choosing a different meaning arbitrarily(or actually, quite deliberately) is disingenuous at best.
Where is the slave
GitLab is about removing the needless ceremony to getting your application deployed.