Apple released iOS 14.0.1 last Thursday, September 24, and as is traditional, Apple has now stopped signing iOS 14, making downgrading to that version of iOS no longer possible.
iOS 14.0.1 was a bug fix update that fixed the following issues:
Apple is known to routinely stop signing older versions of operating systems after it releases a public update, as part of an effort to encourage users to keep their devices’ operating systems up to date.