reviewtasks is sanitized for deleted posts. This means that it is hard, if not impossible, for the users that care and for those that want to evaluate the effectiveness of a community to find reliable statistics from SEDE on completed reviewtasks.
If the data was available it would make it possible to answer this question for example.
reviewtasks table be excluded from the process of removing rows that link to deleted posts? If that is not feasible can an extra table be introduced like
reviewtaskswithdeleted similar to what is done with
postswithdeleted, as suggested by gnat.