From T328032#8936008:
We acknowledge the fact that if someone registers, their answers are aggregated, and then they unregister, their answers will remain in the aggregated data even if the person didn’t participate. We still don't know if this will be stated explicitly somewhere in the interface.
This could be either in the form of a documentation comment in the code, or explained in the interface. Determining that is part of this task.
Acceptance criteria and design TBD.