Or perhaps I was the one who was drunk

synapse/http/matrixfederationclient.py

@@ -193,7 +193,6 @@ def _send_request_with_optional_trailing_slash(
             self,
             request,
             try_trailing_slash_on_400=False,
-            backoff_on_404=False,
             **kwargs):
         """Wrapper for _send_request which can optionally retry the request
         upon receiving a combination of a 400 HTTP response code and a
@@ -205,9 +204,6 @@ def _send_request_with_optional_trailing_slash(
             try_trailing_slash_on_400 (bool): Whether on receiving a 400
                 'M_UNRECOGNIZED' from the server to retry the request with a
                 trailing slash appended to the request path.
-            backoff_on_404 (bool): Whether to backoff on 404 when making a
-                request with a trailing slash (only affects request if
-                try_trailing_slash_on_400 is True).
             kwargs (Dict): A dictionary of arguments to pass to
                 `_send_request()`.
 
@@ -585,7 +581,7 @@ def put_json(self, destination, path, args={}, data={},
         }
 
         response = yield self._send_request_with_optional_trailing_slash(
-            request, try_trailing_slash_on_400, backoff_on_404, **send_request_args)
+            request, try_trailing_slash_on_400, **send_request_args)
 
         body = yield _handle_json_response(
             self.hs.get_reactor(), self.default_timeout, request, response,
@@ -701,10 +697,11 @@ def get_json(self, destination, path, args=None, retry_on_dns_fail=True,
             "retry_on_dns_fail": retry_on_dns_fail,
             "timeout": timeout,
             "ignore_backoff": ignore_backoff,
+            "backoff_on_404": False,
         }
 
         response = yield self._send_request_with_optional_trailing_slash(
-            request, try_trailing_slash_on_400, False, **send_request_args)
+            request, try_trailing_slash_on_400, **send_request_args)
 
         body = yield _handle_json_response(
             self.hs.get_reactor(), self.default_timeout, request, response,