Legacy Forum Thread URLs Deprecated in 12.1.3?

Related to  RE: Error Processing File Viewers / Unable to Format URL for page forum_thread_legacy values , it now seems that in 12.1.3 these types of URLs

sitename.com/group/f/###/t/#####

  no longer resolve at all. Instead they raise an Object reference not set to an instance of an object error

Did I miss where these legacy format URLs were fully deprecated in the release notes? Is there a method to generate the appropriate redirect table rather than erroring?