See: https://www.truthinaccounting.org/library/doclib/Financial-State-of-the-Union-2020.pdf
"A new analysis of the latest available audited financial reports found the U.S. Government’s overall financial condition worsened by $9.84 trillion in 2020. Our measure of the government’s financial condition includes reported federal assets and liabilities, as well as promised, but not funded, Social Security and Medicare benefits. Our analysis includes $55.12 trillion in unfunded Medicare benefits and $41.20 trillion in unfunded Social Security benefits."
The total according to the above analysis is $123.11 trillion as of 9/30/2020 and this number goes up about $10 trillion/year.
The reason why the Treasury leaves these off the federal balance sheet is that Congress could pass laws to reduce or stop future benefits at any time. But how likely is it that Congress would abolish Social Security and Medicare?