the user-agent string has an essentially arbitrary format, so its not really a good place to be reading data from, or worse serving a different response based on the value.
Citation needed
They won’t.
But also, what exactly were you expecting out of a citation? A survey done about whether people spoof their UA or not? That doesn’t exist and likely never will.
If you have a website, you want to collect statistical data on which browsers are accessing your website and test for the best experiences on the top browsers.
what REALLY happens is the most popular browser and even version is determined, and anything thats not that doesn't get tested, and eventually breaks. its the pinnacle of lazy programming. "lets use telemetry so that we can point to data that allows us to make software thats as brittle as possible".
Agreed. Feature detection is the way, not UA sniffing.