Comment by Arathorn

Synapse server is the only server that does the things matrix says it can do. All those others don't. Notice only two of those say 'stable' and only one, synapse, implements the full set of features. Also can't switch between them.

And your links to synapse features ("Please note that, as this feature isn't part of the Matrix specification yet, the use of m.room.retention events for per-room retention policies is to be considered as experimental.") may describe certain synapse functionality but in practice is doesn't work and the db keeps growing as does IO load. The compression thing is an attempt at mitigation because the protocol just doesn't handle it.