There is a sharp drop in new account creations on loginwiki since SessionManager was deployed to group1:
https://login.wikimedia.org/w/index.php?title=Special%3ALog&type=newusers&user=&page=&year=&month=-1&tagfilter=&hide_patrol_log=1&hide_tag_log=1
There are two different mechanisms for autocreation, CreateLocalAccount in CentralAuth and the central login mechanism itself; somehow both seem to be failing.