Topic on Talk:New requirements for user signatures

At a minimum, you need to send a wiki notification to all existing users that their "your signature contains markup errors" and "use the verification tool for your signature in the settings." Such verification and sending a wiki notification should be done every time a user logs in to an account or when a signature is automatically inserted. I had to send a lot of messages on user talk pages with the text "your signature contains errors. Because of this, the pages where you leave the signature fall into the lists of pages with errors. Here is a variant of your corrected signature." This should be automatically notified to all old users with a custom signature before they leave a new copy of their signature.

https://tools.wmflabs.org/signatures/ lets editors check their signatures, if they're interested. Getting a list of invalid signatures seems feasible, and we can use MassMessage to leave notes on their User_talk: pages. It's probably only worth notifying active editors, though.