I interpret that "currently" there would be some methods for nodes to exclude these "fake public keys" from their UTXO set already, but if the validation was changed, that would not longer be possible.
Kind of-- half of fake pubkeys are indistinguishable from ordinary pubkeys to begin with. But the other half aren't and so outputs that are actually unspendable due to a detectably fake pubkey could be excluded. If there were some relay rule running this check then none would be detectable anymore.
When an output is unspendable no one can tell if you've pruned it or not, so you're free to use whatever complicated schemes you want to tidy your own utxo set.
Of course spammers, if they really want to be sure that their data remains on the blockchain,
Well that isn't what they actually want. They want to hype their worthless token over some other worthless token, once they've pocketed the victims money they sail off into the sunset and the long term is irrelevant. So it's just a marketing point.
could already now develop a completely "safe" protocol,
Nah, because anyone paying attention would know that the existence of utxotree shows that no such safe method can exist, even using spendable outputs.