I suppose one could argue that IDs and comments left on a deleted user’s observations fall into the same category. A deleted account takes all of those with them. Which all goes to a previous long discussion here:
https://forum.inaturalist.org/t/dealing-with-account-deletion/93
That topic dealt mostly with the loss of content created by the deleted user. But your situation goes to the other side of that coin – content created by ongoing users being deleted without their knowledge or permission, due to someone else leaving the site.
To me, that is one of the strongest arguments for keeping and merely anonymizing content of deleted users – when consistent with applicable laws, and when it would infringe on the content of other users.