support changing the default wiki branch name
in #221159 (closed), it was made possible for the GitLab wikis to inherit the default branch configuration for the site / group / project etc. But it doesn't clearly show a way to change the existing branch name in a wiki. There was an issue filed to support migrating branch names (#333554 (closed)), but it was closed as a duplicate of the former, so it seems worth reopening this discussion in a new issue to avoid any confusion.
Proposal
It should be possible to rename the default branch in existing Wikis.
I am not familiar enough with GitLab's internal to make any sort of technical suggestion here, but I suspect this is related to the &2381 (closed) epic, because I was following #36865 (closed) which turned into that epic (and I couldn't figure out why else I was following that issue).
It could just be that i'm missing something and this is actually already possible, but I doubt it. Right now it's possible for new wikis to inherit the default branch name, as explained in Configure a default branch for your wiki (fixed up in !66404 (merged)).
but that links to Configure a default branch for your wiki which only says the branch name is inherited from the project. then it says:
For any version of GitLab, you can rename this default branch for previously created wikis.
... but that branch rename procedure is the general "how to rename the default branch in a git repository" procedure which doesn't work for wikis because they don't have that same UI.
in fact, for a project with a wiki and not a repository, that UI explicitly says:
A default branch cannot be chosen for an empty project.
so you can't actually change the default branch in a wiki right now, as far as I can tell.