From a February 19 letter to the Commissioner, by the chairman of the House Ways and Means Committee:

Yesterday, we learned that the IRS sent approximately 260,000 CP59 notices to
taxpayers who the IRS claimed did not file a 2019 tax return. The IRS continues to have
a staggering backlog of unprocessed 2019 tax returns that, most recently, was reported to
include nearly 7 million unprocessed individual tax returns. In light of these severe
processing delays, it is very likely that many taxpayers receiving CP59 notices already
filed the returns that the IRS claims are outstanding. For taxpayers who have dutifully
complied with their filing obligations, these notices impose unnecessary stress and sow
confusion. For IRS employees, these notices create unnecessary work while they
struggle to meet the current demand.

https://waysandmeans.house.gov/sites/democrats.waysandmeans.house.gov/files/documents/2021.02.19%20R...