Apple has stopped signing iOS 14.2 and iOS 14.2.1, which prevents users from downgrading to an earlier version of iOS 14.3. It also leaves iOS 14.3 as the only version of the program available to the public.
While iOS 14.4 is now available to developers as well as to subscribers of the public beta program, it will likely take weeks before this version is available to anyone who wants it. Until that happens, iOS 14.3 is the only version of iOS that can be installed on iPhones. Likewise, iPadOS 14.3 is the only version of iPad software available as well.
Apple iOS 14.3 version brought with it support for the ProRAW photographic format, allowing people to edit their photos in more advanced ways while still taking advantage of the iPhone’s computational features. ProRAW is only available to people using iPhone 12 Pro and iPhone 12 Pro Max.
Apple tends to stop signing older versions of iOS when it wants to make sure users are running a specific version or later, often due to bugs or security fixes. I expect it is only a matter of time before the iOS 14.3 signature stops once iOS 14.4 is released to the public.