The RegioneratorChunkDeleteEvent is fired when a chunk is wiped. Note that the event can be called up to 1024 times per tick per cycle running, and Regionerator itself, while much more efficient than it used to be, is still quite heavy. Caution is advised for anyone using it.
Regionerator also makes no guarantee that chunks are fully deleted - it's possible that chunks could be stored unloaded and waiting to be written when Regionerator visits them.

The config option chunks-per-wipe-cycle has been removed as part of the efficiency improvement - a single read and single write is far more efficient.