Apple typically closes the signing window for older versions within a week or two of a new release for several key reasons: About iOS 14 Updates - Apple Support (AL)
You can no longer download and install iOS 14.1 as a fresh update or a "clean" restore.
Apple has officially stopped signing for all compatible devices . This move is part of the company’s routine maintenance of its ecosystem, occurring shortly after the release of subsequent updates to ensure users remain on the most secure and feature-rich software available. What "No Longer Signed" Means
This practice ensures that devices do not run older code with known security vulnerabilities. Reasons for the Decision
When Apple stops signing a specific version of iOS, it means the company’s servers will no longer provide the necessary (digital signature) required to install that version on an iPhone or iPad.
If you have already updated to a later version, such as iOS 14.2 or iOS 15 , you cannot revert your device to iOS 14.1.