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The relay-policy distinction is useful, but the old free-transaction allowance did change. Bitcoin Core 0.15 removed the remaining coin-age-priority handling, including the free-relay options. A zero-fee transaction can still be valid under consensus; getting it relayed or selected by a miner is a separate question. So I'd separate three things here: whether a transaction is valid, whether a node relays it, and whether a miner includes it. The removal is documented here: https://bitcoincore.org/en/releases/0.15.0/.
The fee does the heavy lifting, but it is not the whole story. Bitcoin has never actually removed Satoshi's free allowance, and spam still does not win because relay policy drops too-small or too-dusty transactions before they ever make it to a block. So the fee is the price signal, and relay is the spam filter.
On SN it is the same two layers. The stack cost is the obvious one, but the quieter filter is that an account and a posting history have effort behind them. Both matter, and neither is foolproof, which is why the answer to spam is usually "raise the cost a little" rather than "one perfect fix".