I still stand by that the error message could be a lot clearer regardless.
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.
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.