Sliding Sync is just your old bouncer with lip service and a fancy name, because the Matrix protocol is now officially deemed hopeless to be implemented efficiently on the client side, hence the need for a proxy to maintain your "client state" i.e. your client running on the server.

I'm really tired of the litany of buzzwords and excuses that the Matrix team comes up to cover-up this large sunk cost fallacy of a protocol. Nobody needs a distributed graph database and state resolution to prop-up a chat system. It has not proven to have any real-world benefit nor served to build a niche in which Matrix is in any way better than the competition. It's been a failed experiment for about 10 years with no end in sight. At what point does the community wake up to all the nonsense?