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.
All the error message says is to disable extensions or try a different browser. It doesn't say what went wrong and it certainly doesn't say anything about needing specific user-agents, hell it doesn't even recommend which browser to switch to it just says "try another browser". As someone already mentioned it's not the site's job to figure out how your browser loaded the page wrong just to tell you it did.