As per MS MC246703, Supervised Chat blocks restricted users from starting new chats unless an appropriate supervisor is present. When chat supervision is enabled, supervisors aren't allowed to leave chats and other participants aren't allowed to remove them ensuring that all chats are properly supervised. I think there were some organisations that requested this to be enabled for supervising the chats.
This feature can be enabled in any environment, but it is ideal for educational institutions that want to ensure that students can only chat when an educator is present. To get the most from supervised chat, each user in your environment needs to be assigned a specific chat permission role.
This will begin rolling this out in mid-April and expect to complete the rollout by early May 2021.
These limitations are only applied to new private chats that are created after the supervised chat has been fully enabled.
They do not apply to exist private chats, meetings chats nor channels.
Supervised chat is default off. Supervised chat can be enabled through Teams policies or using a PowerShell cmdlet.