This was the jQuery way. It was called Graceful Degradation.

The entire approach went out of style with the advent of single page apps, React, Angular, VueJS, etc.

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.

the shockwave and flash way was simpler.

download the whole app and run it in browser. you can even run it off line!

jQuery and graceful degradation are different things. The vast majority of sites in the jQuery era that used jQuery did not gracefully degrade.