All the support material I have read is about the 'Planner' App, which I presume 'Tasks by Planner and To Do' has replaced.
You are right.
I guess you refer to this article to manage Planner notification. This setting is configured in Planner client. You should log in Planner and open your plan. Then edit notification of this Plan as below.
Besides, if you use Planner in Teams, additional notifications are provided by Teams, where they can also be managed.
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