OP_RETURN: how Bitcoin's data output went from a 40-byte cap to 80 bytes to uncapped, with the blocks to check it

Before 2014, anyone who wanted to put data on chain encoded it as fake addresses, and those outputs sit in every node's UTXO set forever. Bitcoin Core 0.9.0 added OP_RETURN as a prunable alternative, first with a 40-byte cap, raised to 80 in 0.11. That ceiling stood for a decade. Bitcoin Core 30.0 raised the default to 100,000 bytes in October 2025. None of it is consensus, only relay policy.

I wrote this up for ChainQuery with the RPC calls shown so you can rerun them on your own node: block 350,000 next to block 840,000 with getblockstats, the Counterparty, Omni and Open Assets envelopes, and the VeriBlock episode, when one project was pushing up to a fifth of daily transactions through this one opcode. For the short version of what the opcode is: https://www.learnbitcoin.com/glossary/opreturn