That's generous. I always heard people espouse that ideal, but I rarely saw them actually do it. And I never saw it at work.
There were always certain UX requirements that required JS, and that meant the company wasn't interested in testing to make sure it worked without JS. None of their customers were going to use it that way.
Angular, React, etc helped force it further, but they didn't cause it.
I know I always did. CakePHP and Rails made it really easy to determine if a request came from AJAX or direct and you could slightly tweak the response to match the medium.
Agree that most people didn't, but I was always an advocate.