Not sure if this is a bug or feature request; it's a bit of both.

The format of Mediawiki:sidebar is odd, presumably because of its age.
It must've been one of the earliest written parts of mediawiki.

Whatever it is, it's not wikitext, like e.g. Mediawiki: Lastmodifieddat -- which is also displayed separately from the main block.

Not being wikitext means that variables can’t be called in MediaWiki:Sidebar, and even if they could be, said variables then can’t be parsed using parser functions. This means that the sidebar isn't easily customisable e.g. to different namespaces, or for different user groups.

Generally wiki readers have different needs to wiki editors. The former outnumber the latter by orders of magnitude, but the latter are important as they produce content. Both groups should be accommodated.

Propose therefore to switch the sidebar to wikitext.

OK, so it'd probably break downstream things like skins, but these can be rewritten to accommodate. And it'll break things on update time. But that would be a one-off break that would then be robust and future-proof. If you have to break it, might as well break it now. My prediction is that if you give the community the chance to easily program the sidebar, they'll sort it out pretty quickly.

It would also allow users to modify their own links E.g. user:example could have user:example/vector-sidebar and use that to override the defaults without having to use JS/CSS which isn't super user-friendly. But that's for the future.

Please consider, thanks.