https://www.irs.gov/pub/irs-drop/n-21-21.pdf

The Secretary has also determined that any individual with a period of limitations
to file a claim for credit or refund of Federal income tax that absent this notice would
expire on or after April 15, 2021, and before May 17, 2021 (for example, certain
individual taxpayers with claims for credit or refund in respect of their 2017 taxable
years), is an Affected Taxpayer.