It adds an entire transaction! Every time it does that, it uses up farmore space than an 80-byte OP_RETURN.
So your conclusion is that it's not spam if it looks like a normal transaction to you?
If OTS ever implements sign to contract, it could just be an ordinary 1in 1out payment, which would be less than a tx with a 40 byte opreturn. https://github.com/opentimestamps/python-opentimestamps/pull/14
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