Based on the information in the link, I would assume that this value represents the "Former user" value.
The user's name and User.Id are deleted. Microsoft Planner stores the user's name and User.Id against the TaskUserID. When a user is deleted, Microsoft Planner retains the TaskUserID, but deletes the user's name and User.Id. This enables Microsoft Planner to:
Rename the user to "Former user." All deleted users will be renamed "Former user."
Keep information on "task created by", "task assigned to", and "task completed by" under the name "Former user."
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