They are long deprecated feature. Probably move to an extension if needed by someone.
I'm aware of $wgReadOnly and such; what I'm not aware of is how exactly its existence obsoletes the very useful special pages. At ShoutWiki we regularly use those special pages for locking and unlocking wikis, as using an on-wiki special page does not require server access and it's thus theoretically possible to even create a user group which can (only) lock wikis without giving the relevant users server access. (Plus, let's be fair, even if you do have SSH access, special pages are so much easier to use for tasks like these.)
"Note that this is deprecated, and the siteadmin permission is not granted to any group per default."
In addition, I propose to remove them from MediaWiki core and split it to a new extension regardless whether it is deprecated.