The risk you allude to always existed (and probably will always exist) because 1) it was always possible to mine a block with illegal content in an OP_RETURN, and 2) illegal content can always be embedded in the blockchain using various other methods (even if your BIP is adopted).
Fortunately, there appears to be pretty broad consensus among experts in law that this is not a real risk, anyways: 

Protos
EXCLUSIVE: Lawyers call Bitcoin Core v30 CSAM concerns ‘overblown’
Bitcoin Knots supporters have argued that a change to an upcoming release of Bitcoin Core could open node operators up to legal attacks.