Actually, we're not in uncharted territory, it's just that the 2020 law says the same thing as the 2008 law,  and in 2008 the IRS told people to keep the funds paid to decedents.  The chart was there, it was just redrawn.  The courts may end up saying the same thing this year as was done in 2008, especially for decedents who died in 2020.  But for those who ask "how high?" when IRS says "Jump!", the instructions were as follows:

Return the entire Payment unless the Payment was made to joint filers and one spouse had not died before receipt of the Payment, in which case, you only need to return the portion of the Payment made to the decedent. This amount will be $1,200 unless adjusted gross income exceeded $150,000. If you cannot deposit the payment because it was issued to both spouses and one spouse is deceased, return the check as described in Question 54. Once the IRS receives and processes your returned payment, an Economic Impact Payment will be reissued.

I would not bet on IRS reissuing the check, because they are making things up as they go along.  Nor would I assume that the surviving spouse can claim the $1,200 credit on a 2020 return next year, because IRS computers may still show the payment as having been issued.