Completed prospect plan steps get added as interactions on the constituent record. However, if the prospect plan step is deleted, the orphaned interaction remains on the constituent record.

My suggestion is that one of the following should happen when deleting a prospect plan step:

1) The constituent interaction should automatically get removed when the prospect plan step it originated from is deleted.

OR

2) Alternatively, short of an automatic deletion, when deleting the prospect plan step, there should be a prompt that asks the user if they want to delete the associated constituent interaction too.