And one of the devs kindly looked it over to make sure that I got the details correct. :-D
Please take a look at New requirements for user signatures#Outcome and let me know if you have any questions.
The first stage (the point at which people will stop accidentally creating broken signatures) could happen on the normal wikitech:Deployments train that will happen from Tuesday, June 30th to Thursday, July 2nd. The end of "grandfathering" (when invalid signatures will stop being added to pages) is at least several months from now.