It's hard to differentiate between completely disabled JavaScript, some individual JavaScript files being blocked by an (overzealous?) ad blocker, and the browser not implementing some required JavaScript feature.
I've been using NoScript for about a month full-time.. and it's insane the things you can leave blocked and still see a site as it's meant to be. Ads, trackers and internal tools that somehow I have to contribute my data to, and their accompanying libraries EASILY make up 70% of all JS.
> Ads, trackers and internal tools that somehow I have to contribute my data to, and their accompanying libraries EASILY make up 70% of all JS.
Citation needed. Literally 100% of the JS code i've written over the past 24 years has been 100% free of "ads, trackers, and internal tools that somehow users have to contributor their data to."
> Citation needed. Literally 100% of the JS code i've written over the past 24 years has been 100% free of "ads, trackers, and internal tools that somehow users have to contributor their data to."
It's a fair assumption that the majority of the code on the internet probably isn't written by you, so what you or any other individual writes isn't exactly a counter-argument.
I'd assume the poster you replied to was referring to the Javascript that gets delivered to their browser on a day to day basis by general purpose web sites, for which a significant percentage being ads and related unwanted content is entirely plausible.
If you were to capture all of the Javascript delivered to a randomly selected person's browser during a normal day I would easily believe somewhere between 50 and 80 percent of that Javascript was things that if the user was given a real choice they would not choose to load.
And on the flip side you can trivially detect if some .js file was blocked from loading with some inline <script> tag that checks for whatever the .js file should expose.
The third case with the browser not implementing some feature is a bit more work but usually also rather easy to do.