First I want to note that I'm not a fan of putting this on display in the bottom of a locked filing cabinet stuck in a disused lavatory with a sign on the door saying Beware of the Leopard. I only saw it because it accidentally turned up in the middle of a list of Phab search results.
The link requirement is helpful to users, the error message would be clear and simple, and it's surely in line with policy or practice at probably every wiki.
The linter portion is not remotely supported by the why rationale offered for the proposal. It would pointlessly disrupt users with confusing error messages on perfectly valid wikitext. It would be insane to suggest things like
<small><sup>this</small></sup> could or would ever be banned. @Whatamidoing (WMF) Please ping me if linter is dropped from the signature proposal. In that case I can skip incorporating this signatures item in a linter-related discussion planned for the new EnWiki Village Pump page.