As of CHT v3.9.0 (#6181), tasks are purged from the phone 60 days after the task has been moved into a terminal state (“Cancelled”, “Completed”, or “Failed”). This is done in Sentinel so it relies on the phone syncing the status update to the server, then being marked to be purged, and then syncing back to the phone.
A task in the “Ready” state won’t be purged. However if the due and end dates are in the past then the state should be updated on the phone, which will mean they do get purged eventually.