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.
Because this entails quite a bit of work and potential breakage in many overlapping systems, we are going to make this change as we migrate projects to GitLab, and not before.
This will enable us to combine the considerable effort of changing Git remotes for both projects in tooling, documentation, and the developer community's local checkouts of repos, and verify changes as we go.
In general it's hoped that the migration to new GitLab remotes and an entirely new CI system will account for most of this work, but there are bound to be some naming-specific problems. We can use this task to track issues arising from the rename, hard-coded branch names, etc., as they arise.
Do not pre-emptively change the branch name on any repositories in the mediawiki/ namespace. We understand the urgency of efforts to deal with these issues, and appreciate the community's patience as we tackle them systematically in a setting where they impinge on a lot of complexity and technical debt.