Added Threaded Messaging (#21303)

docs/labs.md

@@ -183,4 +183,12 @@ When enabled, historical messages will use the current profile for the sender.
 
 ## Pin drop location sharing (`feature_location_share_pin_drop`) [In Development]
 
-Enables sharing a pin drop location to the timeline.
+Enables sharing a pin drop location to the timeline.
+
+## Threaded Messaging (`feature_thread`)
+
+Threading allows users to branch out a new conversation from the main timeline of a room. This is particularly useful in high traffic rooms where multiple conversations can happen in parallel or when a single discussion might stretch over a very long period of time.
+
+Threads can be access by clicking their summary below the root event on the room timeline. Users can find a comprehensive list of threads by click the icon on the room header button.
+
+This feature might work in degraded mode if the homeserver a user is connected to does not advertise support for the unstable feature `org.matrix.msc3440`  when calling the `/versions` API endpoint.