When someone is assigned to a task, they cannot unassign themselves without it being added to the conversation's correspondence. This automatically subscribes them to the correspondence of the task that they just unassigned themselves from, which frankly is pretty counterproductive.
While I love Planner, it's ridiculous that I have to tell everyone "even though you're assigned to this task, the only way you receive correspondence about it in your inbox is by adding a comment to the thread.", as subscribing to view all the correspondence of all tasks going in and out is just an easy way to build a nightmare in one's inbox. Of course, that extra required step breaks our task flow over and over and over again, and forces me to spend extra time chasing people down with duplicate information. There needs to be a way to not just add and remove assignments to a task card, but to add and remove those subscribed to the correspondence thread.
While something similar might exist in a teams or groups setting, I need it to be within Planner, as an option on each card. Explaining all the extra steps required to make this work gets to be like herding cats once you get over 50 people involved in a board, and we have some big projects. Chaos!