While investigating some Bitcoin coinbase transactions, I notice that a lot of them have an output of the form
OP_RETURN OP_PUSHBYTES_36 b9e11b6d...
where '...' is 32 bytes, probably a hash of something. I know coinbase outputs can be used for whatever the miner likes, so could be arbitrary. However, I do see a lot of examples starting with the exact same b9e11b6d four bytes. Merge mining is one possibility, but with some googling, I could not find examples using this format.
For example, I started by looking at block 733528. It has 3 op-return outputs. The first is for merge mining with RSK, the third is the Merkle root of the signature data (indicated by the 4 bytes aa21a9ed), and the second is the mystery one I am asking about.
The previous block 733527 also has this mystery output as its fourth op-return. The block before that does not have such an output, but the one before that does.
Even going back a few years to block 580000 (created 9 June 2019), I see outputs of this form.
Can someone identify what this is? Is it merge mining some other blockchain, or is there another purpose to these outputs?