2018/2019 whichever is the most recent filed, not sure if that means people will get money they "should" have gotten or not (loss of income/new child etc) when filing 2020

"Are the checks taxable? No, they are not taxable. The only catch is that technically a person’s 2020 income is what qualifies them for the payment. Since no one knows their total 2020 income yet, the government is using tax returns from 2019 and 2018 to figure out who qualifies for a check. It is possible that someone may have to pay back some of the money if his or her income this year turns out to be significantly more than it was in 2019 or 2018. That’s expected to be a relatively small share of people, and the money would not have to be paid back until April 15, 2021."

https://www.washingtonpost.com/graphics/business/coronavirus-stimulus-check-calculator/