Can somebody explain how it helps with the privacy?

Assuming most of the users of the browser are on the latest version, it implies that majority of users are already sharing the same useragent. How does useragent version # helps with reducing the fingerprint?

It’s freezing to a static version, everyone would have the same static (frozen) version from now on, not a build number. So you can’t use it as part of a finger print.

https://github.com/WebKit/webkit/commit/4bc237458115ac916f39...

Also suggests you’d need to do capability detection other ways in future.