Let’s be absolutely clear about this. The proposed invalidation of 64 byte transactions does not in any way represent a fix to a vulnerability. Valid blocks do not have malleable block hashes.
Nor does it simplify the design of mitigating the invalid block hash caching weakness. Just the opposite, as a mitigation it is far more complex and costly than the existing solution. And note that caching of invalid messages as a DoS optimization is itself counterproductive, as discussed in detail.
In terms of consensus, it adds a pointless and counterproductive rule. It will require validation checks that are not necessary. If you want to argue it as an SPV bandwidth optimization, have at it. But please do not perpetuate this error that it fixes something.