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.
You can make BIP first, but Bitcoin Core or other full node software doesn't have to implement such feature.
If I remember it well, some altcoins like NXT have/had(?) such a feature.
I don't know about NXT, but centralized cryptocurrency might have such 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.
I understand your explanation. But full node operator is unlikely to use such feature and feature specific for Bitcoin Core doesn't need BIP.
"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.
That's possible, but don't forget it's more expensive since they doesn't get witness "discount".