If I'm reading the IRS data[1] correctly, "debts and mortgages" are considered a deduction on the estate valuation, which means any money left in the estate (i.e. instead of in a trust) solely to cover loans would not be taxed. I think the idea is that you would roll the debts until death, at which point the estate can sell the securities with their stepped up cost basis, thereby avoiding (nearly all) capital gains tax.

I'm not an expert on this, and I could be misunderstanding some subtlety here.

[1] https://www.irs.gov/statistics/soi-tax-stats-estate-tax-fili...