OP_RETURN outputs can include up to 80 bytes of data which is permanently recorded on the blockchain. These OP_RETURN outputs can’t be spent and are typically zero-value outputs. However, some transactions have an output value higher than zero. These bitcoin are permanently unspendable and thus provably lost. The article How Many Bitcoins Are Permanently Lost? (2019) dives deeper into the topic of provably lost bitcoin.
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