https://protos.com/exclusive-lawyers-call-bitcoin-core-v30-csam-concerns-overblown/
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: https://protos.com/exclusive-lawyers-call-bitcoin-core-v30-csam-concerns-overblown/
Discussion
“The shift in relay defaults does not materially increase the risk that CSAM enters the system — it simply affects propagation. The larger legal challenge lies in educating policymakers: the Bitcoin protocol was never designed to police the nature of arbitrary data, and pushing responsibility onto node operators misunderstands both the architecture and the intent of OP_RETURN.”