My second EIP was deposited by the IRS in 2021, not 2020. Should I consider the EIP amount for tax year 2020 or 2021?

I do believe you only add in what you received in 2020. 2021 will be added to 2021 tax questions if received.

EIPs are not taxable income. there is an economic impact payment form in the 2020 tax return that covers EIP 1 and 2. TurboTax should ask questions about these.  if you don't enter the 2nd EIP even though you received it in 2021, your refund will be overstated. then the IRS will lower your refund because its records will show it was received.