As part of T254646 it's recommended to not use "master" as a term, even by itself (see https://inclusivenaming.org/word-lists/tier-1/_master/). We should use another name in our repos for the default branch.
The most common replacement seems to be "main", see e.g. T281593 and https://github.com/github/renaming. The latter also have instructions for how to do this on Github.
The default for new WMSE repos has been set to "main". So this affects pre-existing repos. We can probably limit ourselves to the ones which are actively maintained.
I've gone through the WMSE repos and changed "master" -> "main". Some already had "main" and one was archived, in which case you can't rename the branch. Github is taking care of all the things on Github, such as pull requests. Links should also be redirected.
One thing that you need to do manually is to update any local clones of the repos. There's a popup for that when you visit a repo that had it's branch changed recently (not sure exactly how long that will show up). Same instructions are found on Renaming a branch. @Lokal_Profil, @Alicia_Fagerving_WMSE, @kalle, you may want to update your local repos or at least save the link for when you work on them next.
Repos with main branch renamed:
Already had "main":
Not changed because they're archived:
I had a look through the repos using the API and it seems I didn't miss any, so I'd say this is done.