UA sniffing should be a last resort but there have been times where browsers have claimed to support something but had hard to detect bugs and the cleanest way to handle it was to pretend the feature detection failed on those older browsers.
This is increasingly rare – the last time I had to do it was for Internet Explorer – but I would not take a bet that nobody will ever have a need for it again.
As an example, I recently had to deal with an exception to a TLS 1.2 requirement because while almost all of our traffic is from modern user agents, there was a group of blind users with braille displays using Android KitKat which defaults to only supporting a maximum of TLS 1.1 and “go buy an expensive new device” isn’t an appropriate response even though you might be entirely comfortable saying that random internet crawlers getting an error is okay.
