Answer: Set Page Break Preview
In the Page Break Preview view, the unused area will be greyed out.
Enable the sheet in which you want to display working area only, and click View > Page Break Preview. See screenshot:
If you want to go back to normal view, just click Normal under View tab.
Resources:
https://www.extendoffice.com/documents/excel/4679-excel-grey-out-unused-area-cells.html
