The IRS says it will be caught up with all 2020 and earlier personal tax returns soon,
with 2020 business tax returns shortly thereafter:
https://www.irs.gov/newsroom/irs-continues-work-on-inventory-of-tax-returns-original-tax-returns-filed-in-2021-to-be-completed-this-week
Also:
2.1 Million unprocessed Form 1040-X
3.7 Million unprocessed Form 941
215,000 unprocessed Form 941 X