I have been doing web development pretty much since the web was created.
"Sniffing your browser extensions is normal to be able to render the page correctly" is not and was never a sensible argument. 20 years ago, neither Chrome nor the iPhone existed yet. Most people browsed the web on computers, and "works best in Internet Explorer" was widespread. Web developers were lazy and many of them literally only tested their sites in IE on Windows. Browser extensions themselves were much more of a niche thing since IE didn't support them.