From IRC:
~/git/oilshell/oil/.builds (dev/andy-33)$ git push sourcehut (11:26:32 AM) oilshell: remote: {"errors": [{"field": "tags", "reason": "Invalid tag name, tags must use lowercase alphanumeric characters, underscores, dashes, or dots"}]}
https://git.sr.ht/~sircmpwn/git.sr.ht/commit/163edba4015947670d934ad1590dad1fb86f1104
The change seemed to be that branch names are included as build tags
This would be very welcome, many projects I contribute to use slashes in names.