Microsoft Teams activity feed is not just a chat notification aggregator, it combines notifications from your chats and from applications you are using in Teams.
To help filtering between notification you have read and the ones you have missed, you can now toggle your unread activities in one click.
1. Click in the Activity icon from the left-hand menu. Use the toggle, Unread only.
2. All your unread activities will appear filtered and ordered by date in the activity pane, if you don't have any unread message, you will see a message saying You're all caught up!
3. If you want to keep an activity in the unread section to take care of it later after reading it, do the following:
a. Toggle off the Unread only
b. Select message and on mouse hover, click ellipsis ...
c. Select Mark as unread
https://competenz.freshservice.com/support/solutions/articles/5000083304