After iOS 15, iOS 15.0.1 released on October 1. Right after this, Apple stopped signing iOS 15.0. The iOS 15.0 was released on September 20.
Since iOS 15.0 is no longer being signed by Apple you cannot downgrade to that version if you’ve already upgraded your device to iOS 15.0.1.
As per reports of MacRumors, last week Apple also stopped signing iOS 14.8 which was the version of iOS being used before iOS 15.0 came. Therefore the only available version of iOS at this point is iOS 15.0.1. Thus even the ones who’ve upgraded to iOS 15 cannot downgrade to iOS 14.
Apple has always done this to older versions after new versions of the OS have been released. This is done so that customers keep their operating systems up to date.
iOS 15.0 brings up major version update that introduces features such as the Focus Mode, Live Text, on-device Siri, iCloud+, Safari changes, and more.
iOS 15.0.1 brought in a fix for a bug that stops an authenticated Apple Watch from being used to unlock an iPhone 13 when the user was wearing a face mask.
It also fixed a bug that could cause the Settings app to wrongly display a storage full alert along with the fix for a bug with audio meditations for Fitness+ subscribers.