Per T281593:

As part of the effort described in T254646: Reconsidering how we name things, Release Engineering plans to change the mainline branch name on all WMF (and WMF-hosted) Git repositories from "master" to "main". This accords with changes made by platforms such as GitHub and GitLab, as well as work underway on the Git project itself.

I assume that this decision still holds in principle. However, it was previously planned to be implemented as part of the migration of these repositories from Gerrit to GitLab, which is no longer planned. It follows, then, that we need to find some other opportunity to do this migration.

See also: T279612: Gerrit: default branch for new projects should be "main"