ProjectTask predates the faithful Task type and carries its own vocabulary - TaskNumber, TaskTitle, AssigneeSku, EstimateHours - where SharePoint has AssignedTo, Body, StartDate, DueDate and PercentComplete. CrewTask already inherits Task correctly, exactly as Workflow Task 0x010801 inherits Task 0x0108; ProjectTask does not. Aligning it would let one content-type rollup catch every task in the platform. Deliberately deferred rather than done alongside EST.10: it moves 109 live rows, and doing that in the same cycle as the rollup would confound the two changes.