A new feature is just on his way for the Teams client (rolling out expected to be completed by mid-March).
You can now mute notifications when you are in a meeting; helping you to no longer be distracted during the meeting.
There is no policy available to allow administrators to manage it.
To turn off the notification during a meeting, open your Teams client and access the Settings\Notifications\Meetings and calls option
Edit the option for the meetings and calls to mute the notifications during the meeting or a call