531 needs to be amended to clarify its relationship with 1242.
It seems clear to me that the practice right now is that RFCs are used to induct crates into the nursery and promote or deprecate them out of it, but that while in the nursery those crates evolve and churn without any RFC process. The use of RFCs to manage this movement of crates seems contrary to 531 (to be 100% clear I'm not trying to challenge the process at all, I just want to clarify it).
Its unclear to me if, once promoted into rust-lang (as regex seems poised to do), crates would need to undergo the RFC process for API evolution the way std does. 531 suggests not, but it was written before there was a notion of a crate nursery.