That becomes an explicit choice by the site to request more information, it's up to the client/browser how it responds to that. Fewer bits of information are exposed by default.
> JS won't have access to it anymore - TFA wants to deprecate navigator.userAgent, so only the webserver would have access to user agent details? Why?
I should have linked to the top-level repo with the explainer (https://github.com/WICG/ua-client-hints) as it's not immediately clear from the spec, but access to the hint values is provided via getUserAgent()