Bitcoin Update Idea May Expose Wallet Activity as Legacy Privacy Feature Faces Removal

Bitcoin Core PR #35405 seeks to remove the BIP125 replace-by-fee (RBF) signal, though any real privacy gains will depend on wallets aligning around a unified MAX-2 nSequence standard.

Proposed by developer rkrux on June 19, 2026, the change argues that the legacy opt-in RBF flag has become obsolete. With full-RBF now the default mempool policy, the BIP125 signal no longer plays a functional role and instead remains as a redundant on-chain identifier.

The proposal extends beyond routine maintenance. It forms part of a broader privacy push that hinges on cross-wallet coordination to adopt a single nSequence default—an issue far more complex than simply removing a legacy flag.

Under BIP125, transactions were marked as replaceable if any input used an nSequence value below 0xffffffff − 1. Introduced in Bitcoin Core 0.12.0 in 2016, this mechanism allowed users to increase fees on unconfirmed transactions while preserving first-seen mempool behavior.

That signaling has since lost relevance. Following the introduction of full-RBF via the mempoolfullrbf option in version 24.0—later made the default—nodes now replace transactions regardless of nSequence values. As a result, the opt-in flag no longer influences transaction handling.

However, keeping the signal introduces privacy risks. Because nSequence is a required field, wallets cannot omit it. Without coordination on a replacement value, removing the BIP125 flag could lead to distinct sequence patterns, making wallets easier to identify on-chain.

As contributor Murch emphasized, dropping the signal alone does not eliminate fingerprinting. Each transaction input still requires an nSequence value, making ecosystem-wide standardization critical.

Murch and Electrum developer SomberNight have both advocated for MAX-2 as the default, supported by data showing it is already used in roughly 75% of transactions. Other options, such as MAX-1, were dismissed as they would introduce new identifiable patterns rather than blend into existing norms.

There is also a forward-looking benefit. Future upgrades involving nVersion=3 transactions and Package RBF may assign specific roles to MAX and MAX-1, positioning MAX-2 as the most practical long-term choice while reducing future migration friction.

For users, the ability to bump transaction fees remains unchanged. What changes is the visibility of replaceability. Merchants who previously relied on the BIP125 flag to assess zero-confirmation risk will need to assume all unconfirmed transactions are replaceable—reflecting the reality since full-RBF became standard.

If implemented and widely adopted, the proposal could mark a rare moment of coordinated default alignment across the Bitcoin ecosystem, contrasting with the slower, fragmented rollout of full-RBF in earlier years.