Undeprecate /initialSync for peeking (#2036)

Signed-off-by: morguldir <morguldir@protonmail.com>

changelogs/client_server/newsfragments/2036.clarification

@@ -0,0 +1 @@
+The `POST /_matrix/client/v3/rooms/{roomId}/initialSync` endpoint is no longer deprecated, as it is still used for peeking.

content/client-server-api/modules/room_previews.md

@@ -13,7 +13,7 @@ Visibility](#room-history-visibility). setting, along with a special version of
 
 A client wishing to view a room without joining it should call [GET
 /rooms/:room\_id/initialSync](#get_matrixclientv3roomsroomidinitialsync),
-followed by [GET /events](#get_matrixclientv3events). Clients will need to do
+followed by [GET /events](#peeking_get_matrixclientv3events). Clients will need to do
 this in parallel for each room they wish to view.
 
 Clients can of course also call other endpoints such as [GET

data/api/client-server/room_initial_sync.yaml

@@ -9,8 +9,10 @@ paths:
       description: |-
         Get a copy of the current state and the most recent messages in a room.
 
-        This endpoint was deprecated in r0 of this specification. There is no
+        This endpoint should now exclusively be used for "peeking" into viewable rooms
-        direct replacement; the relevant information is returned by the
+        you're not a member of ([Room Previews](/client-server-api/#room-previews)).
+        Using this endpoint for something other than peeking was deprecated in r0 of this specification.
+        The relevant information for syncing is now returned by the
         [`/sync`](/client-server-api/#get_matrixclientv3sync) API. See the
         [migration guide](https://matrix.org/docs/guides/migrating-from-client-server-api-v-1#deprecated-endpoints).
       operationId: roomInitialSync
@@ -161,7 +163,7 @@ paths:
             You aren't a member of the room and weren't previously a member of the room.
       tags:
         - Room participation
-      deprecated: true
+      deprecated: false
 servers:
   - url: "{protocol}://{hostname}{basePath}"
     variables: