The User Agent Client Hints API looks like a very early draft. I could not see any proposed timeline for implementation or estimate of when this might become a supported standard.

I would not personally rely on this as a substitute or replacement for User Agent by September (Google Chrome 85).

Going way back to the original iPhone Web Apps session at WWDC in 2007, they specifically cautioned about the problem of sniffing UA strings.

Of course, the reality of the web meant they had to do a bunch of compatibility hacks to get pages to display well.

(Gecko appeared in the original Safari on iPhone UA, IIRC)