But in first place, who would bother implement such feature? Even if someone decide to implement it and make PR request, would it be accepted by maintainer of certain full node software?
The "make OP_RETURN
completely ignorable" (=not even necessary for block validation) part would have to be of course a BIP. If I remember it well, some altcoins like NXT have/had(?) such a feature.
The notify-and-take-down system however could be a separate software using Bitcoin Core's RPC interface, which could be provided by third-party developers; most likely simple Python scripts would be enough. The only thing Bitcoin Core would have to provide is a command like "delete OP_RETURN data from transaction X", and that would have to be part of the BIP.
The thing is that this would make OP_RETURN data something very similar to a "sidechain" or "extension", and thus not part of the "canonical" chain, so it would be a quite massive change and conceptually very close to simply storing the data on another chain or in an off-blockchain P2P network. "Destructive" attackers wanting to "poison" the blockchain would for sure go for another methods: if Taproot is restricted, then use (as I already wrote) methods like fake addresses and amounts.