I think you’ve misheard or misinterpreted the GraphQL Q&A question. Deleting data is covered through mutation messages.
The question was asking about versioning of the schema, and how you evolve the schema over time. The guidance was that you should avoid deleting or changing the data type of a field. If necessary you should add a new field with distinct name rather than change an existing field.
If I remember right: Lee said they have fine-grained analytics on which fields are being used by GraphQL clients and were on the verge of dropping (deleting) a couple of fields with really low usage. (I think he said ‘epsilon’)
It was interesting stuff, and a great conference!