I'm thinking it could be interesting to know how many times (or in some other useful unit than "times" - how often) a specific replication slot has "blocked" xlog rotation. Since this AFAIK only happens during checkpoints, it seems it should be "reasonably cheap" to track? It would serve as an indicator of which slave(s) are having enough trouble keeping up to potentially cause issues.
Not having looked at that code at all yet, would this be something that's simple to add?