I'm adding this as a "support request" as opposed to a bug because I'm not sure how exactly this might have happened but want to see if anyone has a thought / suggestion.
I'm upgrading a Drupal 7 site to Drupal 9, and there's a user base of about 12k. I'm migrating users, but my migration code isn't currently mapping the "old" mailchimp subscription field to the new one (my understanding is that webhook lookups usually set it for a user in any case).
I have a list subscription checkbox in my user account fields that defaults to "required". When I started my first import post-configuration of this field, I started getting notifications in my migrate script output that people were being signed up as they were being added to the site. Having that default in place made this make some sense, so I immediately stopped the migration so as not to inadvertently subscribe folks.
I then switched the field to be "not" required and proceeded with my import. Everything went seemingly smoothly, but a few days later I heard back from the client that their subscriber base had dropped by ~3k users. This was clearly directly related to my actions based on the timing, but what's even odder is that the affected users didn't seem to just be "unsubscribed", they were no longer present in the audience at all. I ended up exporting some recipient lists from immediately before and after the event to find the affected users and re-import them, and none of them were present in Mailchimp anymore.
It's kind of a long preamble, but I'm curious to know primarily if anyone has any idea how this module might effectively completely delete a record from Mailchimp, even in a case of misconfiguration? Or any other ideas about what might have happened?