We got an answer to the question here.
https://stackoverflow.com/questions/69555640/difference-between-tasks-readwrite-and-tasks-readwrite-shared-in-graph-api-plan
TL;DR Use Tasks.ReadWrite, Tasks.ReadWrite.Shared can be skipped for Planner.
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What is the difference between "Tasks.ReadWrite" and "Tasks.ReadWrite.Shared" permissions in graph API?
From the documentation:
Tasks.ReadWrite:
Allows the app to create, read, update, and delete the signed-in user's tasks and task lists, including any shared with the user.
Tasks.ReadWrite.Shared:
Allows the app to create, read, update, and delete tasks a user has permissions to, including their own and shared tasks.
To me, they sounds almost identical.
On a related matter, can a planner task really be "shared"?
I have not found a way to share a task without sharing the plan. And if you share a plan, all users in the team get access to all tasks in it.
We got an answer to the question here.
https://stackoverflow.com/questions/69555640/difference-between-tasks-readwrite-and-tasks-readwrite-shared-in-graph-api-plan
TL;DR Use Tasks.ReadWrite, Tasks.ReadWrite.Shared can be skipped for Planner.