What is an OP_RETURN output?
A payment output that stores data instead of paying someone. It can never be spent, so nodes don't track it as spendable.
OP_RETURN is an instruction that marks an output as provably unspendable, with room for a piece of data. Services use it to anchor timestamps, proofs and protocol messages in the chain, and nodes can leave it out of their set of spendable coins.
How much data nodes should pass along has long been argued. Bitcoin Core 30.0, in October 2025, raised its default relay limit from 83 bytes to 100,000 and allowed several such outputs per payment. That is relay policy, not a rule change: what a block may contain is unchanged, and operators can set the old limit back.
References
- Bitcoin Core, “Bitcoin Core 30.0 Release Notes”.