According to recent discussions about Ordinals in the bitcoin-dev mailing list, there seems to be some support to make OP_RETURN more attractive to discourage the "misuse" of the Taproot data storage features. So it is possible such a BIP could get support.
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The "make OP_RETURN completely ignorable" feature would not need any centralized elements. Not even a hypothetical "notify and take down" system would need that, although of course those who do the "notifications" need to be trusted from the full node operators.
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It would be a protocol change, because currently OP_RETURN messages cannot (afaik) be completely ignored, they can only be pruned once validation of the block where they're located has been done. So it would need a BIP. Bitcoin Core as the reference implementation would be the first to have to support it. (Don't really get what you mean here.)
nodes that prune are declared as fool nodes not full nodes
because the full blockchain data and verification system are for full nodes. emphasis on the full
other nodes that switch off services like archiving full data and ignoring certain tx formats by just saying they are valid and not going to cause a block rejection if included are the fool nodes
there is no point having this bypass system yet again where a true full node has to keep the data so that it can seed it to leachers where the "ignore" / bypass is just for individual to decide about putting their node into limp/fool mode of not being part of the network security of decentralising the full data set and sullying full verified data to others
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in short. it still ends up true full nodes dont ignore op return and still maintain and contain it