txn_id field in events is called transaction_id

api/client-server/v2_alpha/definitions/event.json

@@ -43,7 +43,7 @@
           "type": "string",
           "description": "Optional. The event_id of the previous event for this state. This will be present only for state events appearing in the ``timeline``. If this is not a state event, or there is no previous content, this key will be missing."
         },
-        "txn_id": {
+        "transaction_id": {
           "type": "string",
           "description": "Optional. The transaction ID set when this message was sent. This key will only be present for message events sent by the device calling this API."
         }

api/client-server/v2_alpha/sync.yaml

@@ -253,7 +253,7 @@ paths:
                         "$74686972643033:example.com": {
                           "sender": "@alice:example.com",
                           "type": "m.room.message",
-                          "unsigned": {"age": 124524, "txn_id": "1234"},
+                          "unsigned": {"age": 124524, "transaction_id": "1234"},
                           "content": {
                             "body": "I am a fish",
                             "msgtype": "m.text"