The 'new' time-based expiry for caches in Synapse seems to have helped out with memory usage (empirically, based on what people are telling me).
It seems like it would be worth enabling by default so that people don't have to know about this obscure option to get the benefits.

Related: #11641 (it's not obvious that even though Synapse's memory usage only grows by default, it is capable of trimming back down).