Make pathological transactions with more than 2500 legacy signature operations non-standard

The selection of this specific figure, 2500, was meticulously chosen to ensure that no currently standard transaction, which would not be considered pathological, is invalidated under today's Bitcoin Core policy standards. Importantly, it's highlighted that while no such transactions have historically violated both the BIP54 sigop limit and current standards, the theoretical possibility exists for the creation of a standard but pathological transaction that would not comply with BIP54. This scenario presents a significant risk of denial-of-service (DoS) attacks against miners who have not upgraded their systems if BIP54 were to be activated under these conditions.

To mitigate such risks, a proposal has been put forward to classify these potentially problematic transactions as non-standard. This move would effectively delay the activation of BIP54 until there's substantial confidence that the majority of the network's hash rate would not include such transactions in a block. This approach is advocated as providing stronger assurances should BIP54 activation be considered in the future, especially since these specific types of transactions have never been utilized and do not serve any practical purpose beyond making validation processes more expensive.

To implement this safeguard, Bitcoin Core PR 32521 has been introduced, aiming for inclusion in the upcoming version 30.0 release and also seeking backporting to previous versions. This proposal underscores a proactive step towards ensuring network security and stability by preventing potential DoS vectors before they can be exploited, reflecting a cautious and measured approach to protocol upgrades within the Bitcoin development community.