It has been over six years and I’ve accomplished solely too little analysis to state the under with confidence, however what I can roughly piece collectively shortly between cursory analysis and from what I bear in mind:
There was an enormous debate concerning the deserves of RBF since no less than 2013, when Peter Todd and John Dillon supplied a Full RBF patch. Gavin Andresen described it as anti-social and amongst others pushed for a First-seen-safe RBF variant which might require a alternative transaction to incorporate all outputs of the changed transaction. It was identified that FSS RBF would solely defend transactions that solely spent confirmed inputs (since transactions that spent unconfirmed inputs could possibly be invalidated by changing the guardian) and that every alternative would add one other change output to the transaction including UTXO bloat.
Regardless, the total RBF proposal noticed sturdy push-back, particularly by a lot of companies constructed round zero affirmation funds (IIRC e.g. Satoshi Cube, BitPay). The 2015 compromise was then to require transactions to opt-in to Full RBF and to require that replacements pay not solely extra charges than the changed transactions however greater than their whole descendant tree to curb DOS assaults. Normal transactions have been left unchanged of their conduct.
Clearly, RBF is only a mempool coverage and can’t be enforced, so nothing would forestall nodes (and even miners) from individually switching to working a Full RBF mempool coverage.
Increasing on this in 2024: Per widespread demand, a startup possibility mempoolfullrbf was launched in 2022 with Bitcoin Core v24.0.1. Nodes with this startup possibility will settle for replacements for any transactions whether or not the unique has signaled replaceability or not.
Beginning with Bitcoin Core v28.0 mempoolfullrbf launched in 2024, this selection is on by default after it was adopted by a supermajority of the hashrate, making it the defacto anticipated conduct on the community. The startup possibility was deprecated and might be eliminated with Bitcoin Core v29.0.