No, your corporation is not required to e-file for TY 2021 if your total assets drop below $10 million. According to Treasury Regulations § 301.6011-5(a)(1) and (f), a corporation must e-file its Form 1120 if it is required to file at least 250 returns during the calendar year ending with or within its taxable year and it reports total assets at the end of the corporation’s taxable year that equal or exceed $10 million on Schedule L of their Form 1120. Since your corporation's total assets dropped below $10 million in 2021, you are not required to e-file the 2021 return.
Sources:
Publication 4163 (12/2023)
Publication 4164 (10/2023)
§ 6011. General requirement of return, statement, or list
IRM Part 21. Customer Account Services. Chapter 7. Business Tax Returns and Non-Master File Accounts. Section 4. Income Taxes/Information Returns
