Freezing the user-agent string won’t prevent sniffing, it’ll just possibly make sniffing of precise version numbers a bit more difficult (… because you’ll be abusing feature detection to achieve it); but for this sort of thing you’re mostly just caring about the platform at a coarse level, and maybe the browser (though I think Edge lost its distinctive <select> style and behaviour in the Chromium migration, so that now all browsers on Windows work roughly the same). So UA freezing won’t cause any trouble at all for this sort of thing.

For various sorts of keyboard shortcuts, this style of platform detection has long been essential, so that you can use altKey/⌘ on Apple platforms and ctrlKey/Ctrl everywhere else. Ain’t never been no feature detection candidate for that.