We would like to have a data source for historical query plans and plan changes.

Recording the time a new query shape appeared, or the time an existing shape acquired a new plan, would be very helpful in diagnosing sudden shifts in database behaviour (e.g., the ability to state unequivocally that a new query shape first appeared just before a performance disruption).

recording new plans in the mongod log may be adequate. Maintaining an in-memory collection summarizing these and including other useful statistics like execution counts, average latency, average time waiting on locks, etc., would be even more useful.