Participants in Teams meetings can use the meeting chat to discuss topics, post pointers, and so on. The access rules are changing for some added to a meeting to make sure that people don’t get too much access to meeting resources (like chat) once the meeting finishes. It’s a small but important point for Teams administrators to understand.
You can disable Chat for Teams users, but is this a good idea? Chat is an integral part of Teams and disabling it seems like a bad idea for many reasons, not least being compliance as all you’ll do is drive users to find another way to communicate – like WhatsApp.