How common? Dunno. People here talk about it whenever it comes up. As a website for chit-chatting about tech stuff we have an unusually large population that cares about this kind of stuff, though.
Why not?
That defeatism brings you nowhere and this is solvable too by using a custom rom and preferring web browser over apps.
With NoScript, and some basic expectations for web designers to try to honour that "graceful degradation" concept that we were all assured was definitely a thing many years ago.
> Is it common for people to disable JavaScript?
https://addons.mozilla.org/en-US/firefox/addon/noscript/ claims about a quarter of a million users. Which I suppose is really not very much in the grand scheme of things. But there are also people getting similar effects using uBlock and other such tools.
But I don't care how popular this is or isn't. It's my computer and this is how I operate it. And I advocate strongly for others to follow suit, because there are numerous advantages.
And empirically, failure of a site to follow this principle tends to correlate with being unhealthy or unbearable in other ways.
There are of course countless exceptions, which I allowlist with a couple of clicks. But overall I find browsing this way to be a breath of fresh air.