The American Rescue Plan Act of 2021, "the Biden stimulus package" is not law, as I write this. It seems very likely to pass though and some people may want to take action in anticipation of it. That would be filing your 2020 federal income tax return immediately. Others may want to put off filing that return even if it is ready to go and they are itching to pull the trigger.
I generally don't make recommendations based on legislation that has not yet passed, but the circumstances here are a little peculiar. Regardless, there can be no harm and probably some good in pulling together your filing information now even if you decide to put off filing.
Recovery Rebate To Individuals
The part of the American Rescue Plan Act of 2021 that affects when to file your 2020 return is Section 9601 2021 Recovery Rebates to Individuals on Page 454.
The recovery rebate is framed as a refundable credit against your 2021 tax, which seems to lessen the urgency around it. The credit is $1,400 ($2,800 for a joint return) plus $1,400 times the number of dependents. The credit is phased out based on adjusted gross income beginning at $75,000 going to zero at $100,000 (Double those numbers for joint returns. It is $112,500 to $150,000 for head of household.)
Timing Of Your 2020 Return
What creates the urgency around filing is that the credit will be refunded in advance. It will be trued up on your 2021 return by a reduction for the advance payment, but the reduction does not reduce the credit below zero. So if you get an advance refund of the credit, which some of us would refer to as a stimulus check, you don't have to pay it back if your 2021 income ends up being over the threshold.
So how are the advance payments computed ? They are figured by applying the rules to your 2019 return unless you have already filed your 2020 return. Then they use your 2020 return. This is what creates the urgency. To take a really simple example. Robin who is single has consistently had a six-figure income, but was thrown out of whack by the plague. Robin is now back on track and expects to do as well in 2021 as they did in 2019.