Currently it is not possible to @mention members in comments and notes of tasks in MS Teams.
This would be a great feature!
Squigs Daddo commented
I agree with CY, I use Asana like this all the time. This request appears in multiple threads and is not getting voted up. Please everyone pile on this one to get it noticed. https://microsoftteams.uservoice.com/forums/555103-public/suggestions/40726792--mention-in-planner-tasks-with-notification
I agree!, this feature in Asana was so helpful for a team member to drill into a task immediately and be updated without creating another chat thread.