Enable sliding sync support by default (#17648)

Co-authored-by: Andrew Morgan <[email protected]>

Author: erikjohnston

changelog.d/17648.feature

@@ -0,0 +1 @@
+Enable native sliding sync support ([MSC3575](https://github.com/matrix-org/matrix-spec-proposals/pull/3575) and [MSC4186](https://github.com/matrix-org/matrix-spec-proposals/pull/4186)) by default.

synapse/config/experimental.py

@@ -338,8 +338,10 @@ def read_config(self, config: JsonDict, **kwargs: Any) -> None:
         # MSC3391: Removing account data.
         self.msc3391_enabled = experimental.get("msc3391_enabled", False)
 
-        # MSC3575 (Sliding Sync API endpoints)
-        self.msc3575_enabled: bool = experimental.get("msc3575_enabled", False)
+        # MSC3575 (Sliding Sync) alternate endpoints, c.f. MSC4186.
+        #
+        # This is enabled by default as a replacement for the sliding sync proxy.
+        self.msc3575_enabled: bool = experimental.get("msc3575_enabled", True)
 
         # MSC3773: Thread notifications
         self.msc3773_enabled: bool = experimental.get("msc3773_enabled", False)