when an account is deleted the associated private key used for signing activities is deleted as well. the activitypub protocol does not really define how software is supposed to deal with deleted and recreated actors (which includes users), so some software has been built with the assumption that any change of encryption keys for a user means it’s an invalid signature and will prevent activities involving that user to federate to that software.
this is the important part.
when an account is deleted the associated private key used for signing activities is deleted as well. the activitypub protocol does not really define how software is supposed to deal with deleted and recreated actors (which includes users), so some software has been built with the assumption that any change of encryption keys for a user means it’s an invalid signature and will prevent activities involving that user to federate to that software.