In light of ABCI++ landing, we need to strongly consider deprecating and removing the priority mempool as the same functionality can be provided by implementing an app-side mempool with ABCI++.

Definition of Done

When we've either decided that we still need the priority mempool (in which case this issue can just be closed), or we've clearly deprecated it and scheduled it for removal in a specific future release.

Follow-up here, if we decide to remove it, would be to create a separate issue for tracking its removal.