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Each version of iOS or iPadOS has a signature, technically a piece of code that is sent from Apple’s servers. Once you installed a new iOS and iPadOS through iTunes, the signed one will be added.
The signature for obsolete versions of iOS is deleted two weeks after the release of the latest system update. You can upgrade or downgrade to any version of iOS that is still signed, but you won’t be able to do so if the version of iOS you want to install is no longer signed.
Read this article here on how to Restore Unsigned IPSW without iTunes