By popular demand, all users except guests can now add and remove task watchers. The built-in User role now grants the permission to change task watchlists.
If you are on Enterprise plan and already granted "change task watchlists" permission to users, this change does not affect you.
As usual organizations on Enterprise plan can customize builtin roles and permissions. For example to revert back to previous watchlist editing permissions you can revoke "change task watchlists" permission from predefined User role and grant it to Admin.