Likewise, unless there's an official government source that very clearly says that it's fine to travel on an unexpired but superseded BRP, there's no way I'd risk it.
I'm not sure that A BRP is actually valid once it has been superseded by another one? Might there be a situation where you leave (possibly legitimately) on a still-valid old BRP, your leave is approved while you are out of the country, and then you find yourself not in possession of a valid BRP to re-enter?
Fiancée visa 2014 → FLR(M) Oct 2014 → FLR(M) Mar 2017 → SET(M) Sep 2019. I'm not one of the experts here - The only thing I know for sure is our story, so please double-check any advice I give!