According to the commit that marks it as withdrawn:
All of BIP62's (including the only-new-transactions) are currently enforced
as standardness rules, but it seems hard to push it further. Every new type
of complex transaction may require new extra rules, and some important types
of malleability cannot be addressed by it (for example, a single participant
in a multisig spend creating a new signature with a different nonce).
It seems wiser to pursue normalized txid or segregated witness-based
solutions, which do solve this problem more fundamentally.
