Is there a way to view in routeros how several wifi clients are connected 802.11b 802.11g 802.11n ?

I would like to turn off 802.11b support but i need to find out if there are still clients using it.

Get detailed list of registered wireless clients by issuing the following command: Then analyze tx-rate-set for each client- 802.11b clients only support CCK rates.

Look at the the registration table and the data rate of connected clients, you can deduct by that.
But I believe B is very old and very very slim chance there is still any device around.

Edit: andriys beat to me it