@Mormegil Is there any way that this functionality could be reinstated somehow while also preserving the ability to use "c:" as the interwiki linking prefix for Commons, perhaps by allowing the creation of redirects on pages starting with "C:" and having links follow the redirects if they exist and link to Commons if they don't? In the rare instance a redirect exists when a Commons page needs to be linked the Commons page can still be linked with the "commons:" prefix, so the ability to link to pages on Commons would not be lost even if "C:" category shortcuts were re-enabled. The current "CAT:" for categories is rather unwieldy and only reduces the length of a link by five letters. Even "CT:" or "CG"would be preferable in place of (or in addition to) "CAT:" if there's no way the ability to use "C:" for shortcutting to "Category:" can be restored, since they are shorter and less unweildy to type while using the left shift key. If consensus is needed to implement any of this I would be happy to start a discussion over the matter.

Ok, although I distinctly remember C:SD was at one time a redirect to Category:Candidates for Speedy Deletion. This was a while ago, however...it seems to have been the case up until 2014. See revision here.

Also, for reference, https://en.wikipedia.org/wiki/MediaWiki:Ipb-blockingself is the message seen when an admin tries to block themselves, though I'm not sure what makes it show only when the admin goes to block themselves.