Executive Summary

Apple restricts iOS/iPadOS restoration to signed IPSW files only. An "unsigned" IPSW means Apple no longer validates installation of that iOS version. Free tools exist to bypass this, but they require specific hardware (checkm8漏洞) or saved blobs (SHSH2). No free tool can restore any unsigned IPSW to any device unconditionally.

Security: Apple blocks unsigned firmware to protect users from vulnerabilities. Installing older versions may leave your device open to security risks. Install unsigned IPSW - Tech Support - MPU Talk

Restoring an unsigned IPSW is generally not possible through official or standard free tools because Apple requires a cryptographic signature (SHSH blob) to authorize the installation. Once Apple stops "signing" a version, their servers will reject the restore attempt during the verification process.

Realistic Advice:
If you didn’t save SHSH blobs for an unsigned iOS version, you cannot restore to it on modern iPhones — no free tool can bypass Apple’s signing server today.