When $wgUploadMaintenance is set to true, it should prevent any modifications from the upload repository: uploads (and reuploads), deletions, file renames.
Upon deletion it correctly displays an error stating the repo is in maintenance mode. However, the upload form doesn't give any warning, and uploading a file actually succeeds.
https://www.mediawiki.org/wiki/Manual:$wgUploadMaintenance
Reproduced on MediaWiki 1.30
Okay, I read from the documentation that this setting is "To disable file delete/restore temporarily. " This makes the variable useless.
I don't see a point in allowing uploads while denying deletes/restores, specially since the later usually requires more specific permissions.
From r56215: Note that $wgEnableUploads really should be disabled too or it'll be a bit silly. ;)
I'd expect a centralized way to set uploads to readonly
FileBackend seems to have a ReadOnlyReason[1], but I'm not able to see how to set up easily