IMO, it's more about developers just using Bootstrap without reading the documentation. The examples are pretty well done, they use semantic elements and are accessible.
For example, the Navbar element has a good HTML structure and uses `aria-label`s that announces what's going on to assistive technologies.
Another one is the modal example, it has semantic elements, the `tabindex="-1"` attribute to make it accessible to keyboards and the correct use of the `aria-label` as well.
So, their documentation pretty much teaches you how to use it correctly. I'm not a fan of Bootstrap myself, but I think throwing the blame at it is unfair.
Good to see they improved, i guess :-)
(I probably paid a lot more attention in 3.x documentation than most because of the mistake in a past life of forking Bootstrap 3 for an enterprise project and trying to keep it in sync with upstream. Never again would I consider doing that.)