This is a quote from the book "On Zero Confirmation Transactions" by Chris Pacia
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An “opt-in” version has already been rammed through, without consensus, and with almost no discussion. The opt-in version only enables replace-by-fee on tr
ansactions containing a sequence number less than the maximum. Technically, retailers will be able to detect unconfirmed RBF transactions and reject them (making the fee bump part of it useless)
. So, at least in theory, the opt-in shouldn’t totally destroy zero confirmation transactions. But the intent is clear. The goal is to eventually eliminate the opt-in part, this patch is just ...