Yes, eventually. Right now the sliding sync MSC is still in flux (e.g. we just realised today that it's missing an explicit flag to notify once the client has caught up with the server, rather than guessing via heuristics: https://github.com/vector-im/element-x-ios/issues/1269#issue...). As a result, the implementation (which is in golang) is being kept separate from Synapse for now while we iterate on it; plus it's a separate set of folks working on it. This also means that SS can be used with any existing server (dendrite, conduit etc) as needed.

It'll get added natively to Synapse eventually, but it'll likely be quite a way off.