Well, after my Dad got a CP80 letter for 2020 return and he dutifully mailed off a new copy of his return, I see IRS is now saying: "Due to processing delays for 2019 and 2020 tax returns, the issuance of CP80 and CP080 (Unfiled Tax Return – Credit on Account) notices has been suspended. If you received a notice for your 2019 return and you filed timely, please refile the return. If you received a notice for your 2020 return DO NOT refile."
I suspect when processing of 2021 returns began, the system auto- generated these CP80 letters and now IRS is flooded with more returns it can't process.