The callbacks that you receive via webhooks are simply intended to notify you when events occur -- i.e., by design, each callback contains only enough data to identify the event that triggered the callback.
Update 10/19/2020:
Unfortunately, you won't be able to use the Get Row operation to retrieve data for row after it's been deleted. Therefore, seems like you'd need to somehow write/save sheet data elsewhere as the sheet is modified (e.g., perhaps in response to webhook notifications that indicate row data was added/modified?), then you could query this saved sheet data to get row data as rows that are deleted from the actual sheet (i.e., in response to 'row deleted' webhook notifications).