I graduated from a US university in 2018 and my favorite math course was taught by a professor who told us at the start that the textbook was just his course notes, which he provided a PDF for, and we were welcome to (and encouraged) use the text as open-notes in our exams so long as we had a printed version of the notes.

The class proceeded to be one of my favorite math classes, and it was so incredible to have a text that was deeply integrated to the course--because it was just straight up what he lectured from. I wish more of my courses had been that way.