I’m not sure what proportion of websites include Google Analytics tracking scripts. I’d be guessing. What I do know is that every site I’ve worked on in the last decade or so has made use of it.
The part I feel more confident about guessing is that only a very small minority use anything beyond the most basic out of the box usage. This was true of me until only a few months ago when I was asked if we could track something other than a simple page load.