The filter I'm gathering data on is called minRelayTxFee -- minimum relay transaction fee. Almost every bitcoin transaction pays fees to miners, and most nodes have a standard filter that checks every transaction sent to its mempool to see if it pays a minimum fee. If not, it refuses to relay that transaction. Thanks to most nodes doing this, there is a mitigation for what would otherwise be a significant DoS vector on bitcoin: without this filter, spammers could cheaply DoS the network by sending out millions of 0-fee transactions.

In the op_return wars, people who oppose spam filters have been arguing for a while now that spam filters "don't work" because some large op_returns bypass the spam filters and get into the blockchain via private mempools, or libre-relay. One of the pro-filter side's responses is that these are exceptional cases; only a small percentage of transactions in the blockchain show any evidence of bypassing the op_return filters, and that helps prove that the filters work.

Recently, the anti-filter crowd has a new argument: spam filters "don't work" because some transactions pay less than the standard minimum feerate; they bypass the minRelayTxFee filter. So I created this website to gather statistics about that, and show that the same counterargument applies: only a small percentage of transactions in the blockchain show any evidence of bypassing the minRelayTxFee filter, and that helps prove that the filters work.