Since CRIME in 2012, AFAIK compression with encryption is considered insecure, and the feature is removed entirely in the TLS 1.3 draft. In addition (and because of that), many (most?) client TLS implementations don't support compression (Java, .NET), meaning that a considerable number of PostgreSQL users don't have access to compression.
Does it make sense to you guys to discuss compression outside of TLS? There are potentially huge bandwidth savings which could benefit both WAN and non-WAN scenarios, and decoupling this problem from TLS would make it both accessible to everyone (assuming PostgreSQL clients follow). It would be a protocol change though.
