If you are using UA sniffing to determine if the device has a keyboard it will be wrong a lot of the time; you can’t tell if a Microsoft Surface has the keyboard attached or an iPad doesn't. you’d probably be no worse off trying to detect it by screen width, which is already a defacto way to detect mobile devices anyways, for better or worse.
(The right solution is probably a new kind of API or media query.)
The specific feature this came up recently is whether or not a chat app should autofocus the input box. Ideally, the answer is "yes" if you have a keyboard (lets you start sending messages faster) and "no" if you don't (the on-screen keyboard would cover up the conversation and be distracting).
There's, of course, no way to detect an iPad keyboard, so currently iPads miss out on autofocus.
Discord, incidentally, uses an alternate solution: "don't autofocus the textbox, but instead detect keypresses outside the textbox and manually insert the corresponding letters in the box". This, of course, completely fails for various non-Latin keyboards, such as Japanese: if you type "chi", it will insert 「cひ」 instead of 「ち」.