I came up with a protocol that I think allows a node to prune its utxo set and still count as a fully validating node. So I no longer think it's true that it has to be held in ram -- I don't think it needs to be held at all, though I think the more of the utxo set you "keep around," the faster IBD would be.

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Idea for pruning the UTXO set in bitcoin

Idea for pruning the UTXO set in bitcoin. GitHub Gist: instantly share code, notes, and snippets.