In SUL3 mode, on the local domain, CentralAuth disables most authentication providers via the AuthManagerFilterProviders hook. For login / signup this has the intended effect, since they just redirect to the central domain, and the providers run there; but these providers are also called on autocreation, which cannot redirect. So a side effect of SUL3 is that autocreations which were previously blocked by an authentication provider (e.g. SpamBlacklist) aren't blocked anymore.
In practice this might be fine. Trying to prevent a user from having a local account on some wikis while they already have one on other wikis is not a particularly useful thing to do. It means the user will automatically get logged-out pageviews on that wiki and that wiki alone, which is confusing to say the least. But some wikis did use it, via AbuseFilter, to prevent usernames which are fine in general but not useful on that specific wiki (e.g. because most users of the wiki are unfamiliar with the script it is written in). So maybe that will require some sort of support.
