Update: I received a message from Discourse saying, “Currently, IPs are retained for security reasons, and there is no way to anonymize them directly from the Discourse interface. The team is having an internal discussion on this topic.”
The context was using “Anonymize User” after having unchecked “log anonymizer details”…so even in that case, Discourse still keeps users’ IP addresses. (I also seem unable to remove the registration IP from my own records.)
@Gavin_Hudson, you might be interested to know about this if you didn’t already.