Written to the chain on 31 Jul 2013, block 249,498.
The protocol genesis is the Exodus fundraiser (block 249498, 2013-07-31) — the first metaprotocol on Bitcoin — which long precedes the first text-detectable OP_RETURN marker (block 292316, 2014-03-24). The early Class A/B encodings are markerless so a text scan can't flag them. Not yet chain-verified.
OP_RETURN explicitly marks an output as unspendable and lets it carry a small data payload. Unlike fake-address methods, it does not create a spendable-looking UTXO. That makes it the least deceptive standard way to place small data on-chain.
More about OP_RETURNEvery byte shown here was read from these transactions. Open one in a block explorer to check it yourself.
