If you desperately need an unsigned iOS version (e.g., for 32-bit apps), buy an old iPhone 5s or 6s on eBay. Those support free unsigned restores.
Unauthorized modification of iOS is against Apple’s terms of service. Using these tools may void your warranty (though you can always restore to a signed version to re-validate). Additionally, downgrading can break Face ID, Touch ID, and Apple Pay if done incorrectly. unsigned ipsw restore tool free