https://spec.matrix.org/latest/#room-structure

The content of the messages can be encrypted. Who is in a room and who sent each message is not. See the “shared data” section of the chart.

Encrypting that data would require something like Sealed Sender (like Signal), and that is entirely absent from the spec and any implementation.

Edit: to the people downvoting, this is the literal Matrix spec upon which all the implementations rely. You are asking me to prove the absence of something in it. If you could, point me to the section that comments on the encryption of metadata in the spec. You may not like the answer (I’d love for it to encrypt metadata too!) but that doesn’t change the fact that it doesn’t encrypt metadata at this time.