findDecisionRetiring

The most recent decision that retired propositionId. Never one where it merely survived.

A proposition can survive one collapse and later be retired into a different survivor (fold A into B, then fold B into C). At that point findDecisionForProposition can hand back the first decision, the one where the id survived, and never reach the one that actually retired it. This method looks at the retired side only, and when a proposition was retired more than once, it answers the newest of those decisions.