Originally PEP 8 had 79 characters. Now that was a weird choice so most companies went with 80 instead, including Facebook. You want a low-ish limit because it makes it possible to fit two files side by side on a typical screen resolution. Even if you don't edit like that, you look at diffs like that. More importantly, a low column limit is helpful to disabled engineers who don't have to navigate horizontally so much.

So we used 80. I always felt bad when the linter stopped people from pushing impactful changes because they went over the limit by two characters. So two years ago I set up a "highway speed limit" style warning in flake8 (code B950 in PyCQA/flake8-bugbear). What it does is it keeps your limit intact (for example "80") but doesn't trigger unless you went over by more than 10%. So the limit happens to be 88.

When I was working on Black, I was faced with a dilemma. Should the formatter stick to 80 or be able to "go over" a bit, too, as we would let humans do. I felt like the latter made more sense as the resulting code looks nicer (fewer occasions to break a single line into three or more). Then I remembered Raymond's talk "Beyond PEP8" where he mentions that experience shows "90-ish" is the wisest choice. So I went with it.