This change was made about three weeks ago, and the number of active editors with errors seems to have declined during this time. I suspect that this is mostly about new accounts, which can no longer make these mistakes.
I decided today to wait another ~10 days before trying to contact anyone else about fixing their sigs. Especially on the biggest wikis, I don't want to flood RecentChanges with hundreds of messages.
I've started a central help page at New requirements for user signatures/Help. Please put that page on your watchlist (and help me figure out the simplest, easiest ways to explain all of this).