The ability to mark an individual prospect plan as historical was shipped with Service Pack 34, from Idea 1009. We will continue to monitor this one (global change).

This is still badly needed and a surprising product gap

Great idea! This functionality would provide efficiency doing clean up after conversion.

Yes. We still need to be able to do this from the front end. 

This is extremely helpful in cases of staff turnover

This would be extremely helpful. Issue has been raised multiple times within the 6 months we've been live.