I found this interesting provision in the Regulations under Section 166 (bad debts) –

(2) Specific cases. Subject to any provision of section 166 and the regulations thereunder which provides to the contrary, the following amounts are deductible as bad debts: . . .

(iii) Claim against decedent's estate. The excess of the amount of the claim over the amount received by a creditor of a decedent in distribution of the assets of the decedent's estate may be considered a worthless debt under section 166.

https://www.law.cornell.edu/cfr/text/26/1.166-1