Filter Groups to Show Unread Posts

Groups have a new feed filter, Unread Posts, to get you to unread content quickly. To see this filter, you must be a member of the group. What’s considered read? Posts you’ve liked, commented on, edited, scrolled through, or viewed in detail. This feature is available in Lightning Experience and Lightning communities.

The Unread Posts filter on a group

Chatter REST API and Chatter in Apex support unread posts.