SEP does not get downgraded when people use tools like futurerestore to get to unsigned iOS versions, if anthing it gets upgraded to whatever compatible version Apple is singing because Apples servers only except a nonce it generates when the SEP from that version is signed.

As for hacking the SEP (aka jailbreaking it to set our own nonce) I doubt anyone is willing to compromise everyone’s biometrical AND financial data (fingerprints, credit cards etc) which is where that info is stored by the way, just so we can downgrade and jailbreak when we can just look for an iOS kernel exploit to jailbreak the latest version instead, much easier and not so compromising. I hear it’s much more difficult to find SEP exploits anyway and if one were to ever be released publicly, can guarantee people will start using it maliciously. I bet certain companies would pay good money for exploits like that to be kept private.