The Follow-ups feature in Word Online will be replaced with Comments effective Mar 1, 2021. Comments can be used to notify others and assign tasks by using @mentions.
This was announced via the Message center earlier this month.
What will happen to my existing Follow-ups?
These will be converted to Comments automatically.
How do I use Comments in Word Online?
You can right-click text and select New Comment to create a comment.
You can use an @mention to notify someone in your organization. Once @mentioned, you can check a box to also assign that user a task.
The assigned user then receives an email with a snippet of the task’s context and link to full document. The user can choose to respond to the comment without leaving the email by expanding Add a comment, or they can click Go to comment to go to the full document.
To resolve the task, the user needs to Go to comment and can then check the task off in the original comment’s header as shown below.