Earlier quoted context omitted.
Most graduate school debt is due to master’s degrees and professional degrees (e.g. JDs, MDs, MBAs). The vast majority of PhD programs are fully funded, fully covering tuition and providing a (modest) stipend.
Citation please? For reference, I was in a PhD program in the humanities at the University of Wisconsin, and it was definitely not fully funded.
In STEM fields, 10% of PhD students reported using external (non-university) funds as their primary source of grad school funding; 10% of those externally funded students (1% overall) reported using loans as their primary funding source.
In non-STEM fields, the proportion of non-funded PhD students is higher, at 35%, of which about half (17% overall) used loans as their primary source of funding.
Given that the majority of PhDs in the US are in STEM, it is correct to say that the vast majority of American PhD programs are fully funded.