Humanities departments at universities in the US have been getting smaller, and more professorship slots are moving towards STEM, in general. However, this has come at the end of a tremendous growth cycle in the humanities if you zoom out to the last 50-100 years. It also seems like more "traditional" fields like History and English are being replaced by Psychology, Sociology, and [x] Studies, which seem to have a very different focus.
Even in STEM, only about 10% of PhDs from top-tier universities go on to become professors. A small fraction of the rest end up adjuncting, but a much bigger fraction take the escape hatch into industry. A PhD in history basically only prepares you to be a history professor, though, so I imagine that the escape hatch is much more narrow in that field.
>so I imagine that the escape hatch is much more narrow in that field
Worst case, someone with a PhD in at least most technical fields is seen as at least as educationally qualified as someone with a Master's and there are lots of sciences like biology or chemistry where a PhD is seen as a pretty good degree for an employer outside of academia for a lot of purposes.
A lot of the humanities, less so. If you have a PhD from a good school in English lit, that may suggest certain things (both good and bad) about your qualifications but isn't necessarily a recommendation beyond a good portfolio. Though the same thing could be said about a law degree which a lot of humanities undergrads I've known got and never really used except in very indirect ways.
Even in STEM, only about 10% of PhDs from top-tier universities go on to become professors. A small fraction of the rest end up adjuncting, but a much bigger fraction take the escape hatch into industry. A PhD in history basically only prepares you to be a history professor, though, so I imagine that the escape hatch is much more narrow in that field.