The doctrine of constructive receipt says that you report the income when you actually receive it and are able to use it.
However, I can see the problem in the situation because of the nature of the ACH system. ACH deposits are a two-step process. The sending bank initiates the deposit which tells the receiving bank that money is on the way and gives the receiving bank a chance to verify account numbers or other information. Then within 48 hours, the transaction is settled and the money is actually credited to the receiving bank. Some banks will allow you to access the funds on notice of pending deposit, without waiting for the settlement. (My current checking account does not allow me to access funds until the transaction has settled, but my previous checking account allowed me access to the money as soon as they received the advice of a pending deposit.)
This can create a situation where the deposit is not settled until Wednesday but you have the ability to withdraw and spend the funds on Monday or Tuesday. That may not happen at the end of this year as you think it might, since January 1 is a Friday and the banks are closed, followed by Saturday and Sunday. I suspect your employer will either make the deposits on the 31st, or wait until January 4, and this won’t be a problem.
However, if you are in a situation where you have access to the money before in 2020 but the transaction is not officially settled by the banking network until 2021, I would ignore the doctrine of constructive receipt in this case and just go by whatever your W-2 says. You will have far more difficulty reporting wages that differ from your W-2 for both 2020 and 2021 than simply reporting whatever is reported to you.
