Withdrawing BIP120/121 due to security issues during soft-forks

There is an inherent problem with BIP120, Proof of Payment: If there is a soft fork, a server that verifies PoPs will accept a PoP as valid without checking any of the new Bitcoin rules. For example, a server will be fooled by a segwit transaction, because the server doesn't have a witness to verify and consequently will accept any PoP with an empty scriptSig. Since BIP121 depends on BIP120, that has to go too.

Specification