From the [Synapse 1.43.0 release highlights](https://matrix.org/blog/2021/09/21/synapse-1-43-0-released):
> Asks clients to prefer [room version 9](https://github.com/matrix-org/matrix-doc/pull/3375) when creating restricted rooms ([#10772](https://github.com/matrix-org/synapse/issues/10772)), via the API defined in [MSC3244: room version capabilities](https://github.com/matrix-org/matrix-doc/pull/3244).