To add color: while v0.3.18 introduced IsStandard(), the real inflection point came one release earlier, v0.3.17.

In the months leading up to v0.13.7/18, Bitcoin saw its first “spam” wave: a flood of free (zero-fee) transactions.

In v0.3.17, Satoshi added a minimal-priority requirement for free transactions, shifting mempool policy beyond pure local DoS protection into network-wide anti-spam. Before that, mempool policy only protected an individual node.

The fight against spammy free transactions lasted several years and made mempool policy increasingly complex. It began under Satoshi and grew significantly under Gavin Andresen. As @BitMEXResearch's thread shows, it was Gavin that initially convinced Satoshi that whitelisting known transaction types was a good idea. Satoshi bought in, but wanted to keep the whitelist permissive.

With hindsight, I think that was a mistake. Free transactions would have faded on their own when the baseline fee rate rose. By adding these filters, mempool policy became needlessly complex. Not only do we have to maintain code that’s no longer relevant, but we also opened a Pandora’s box of governance issues around the mempool. It's a "cure is worse than the disease" situation.

One interesting thing the first spam wave and the latest one have in common: both occurred during periods of extremely low fees. It was practically zero fees during the first wave. During the recent wave of on-chain JPEGs, fee rates went as low as sub-1 sat/vB.

Ultimately, none of this matters once we’re past the subsidy era and there’s a healthy fee market. Fighting spam with filters implicitly assumes the fee market will remain weak. If fees had been higher, spam would have been far less of an issue, as data transactions are less economically efficient than monetary ones.

A lively chain with no filters will live. A dead chain with working filters is still a dead chain.

Fees have always been the greatest spam-slaying hero. Time is on our side: the subsidy continues to decline. The most productive way to fight spam has always been the same: to encourage self-custody and increase monetary demand for the base layer.