[BIP Proposal] Limit ScriptPubkey Size >= 520 Bytes Consensus.

The discussion highlights concerns regarding the potential complexities and unforeseen consequences of modifying Bitcoin's transaction scripting capabilities, specifically in relation to off-chain, presigned transactions that exceed the current size limit of 520 bytes for the scriptPubKey. The conversation points out a hypothetical scenario where an intricate chain of presigned transactions could emerge, based on a complex scheme that may only become known after certain changes are implemented. This raises questions about the ability to upgrade old unspent transaction outputs (UTXOs) without risking the confiscation of funds due to these newly revealed schemes.

Further, there's an exploration of how a taproot transaction UTXO, if later presigned to commit back to a Pay to Multisig (P2MS) larger than 520 bytes, could introduce additional complications. The underlying concern is whether ensuring the security of transactions against such unforeseen schemes would necessitate the perpetual tracking of old UTXOs, thereby complicating the protocol and potentially hindering upgrades. However, it's noted that once a given UTXO mixes with a post-update coinbase UTXO, it might no longer require tracking, suggesting a possible mitigation for some of the complexity.

The email also touches upon the broader implications of making protocol decisions under conditions of uncertainty, particularly when it comes to upgrading on-chain UTXOs while protecting against theoretical, unpublished transaction schemes. It questions whether Bitcoin Core has previously faced similar dilemmas of balancing acceptable risks with the need for protocol improvements. Moreover, concerns are raised about future opcode cleanups and the potential for issues arising from presigned or committed transactions that may rely on opcodes rendered invalid by consensus changes. This reflects a general apprehension about the challenges of evolving the Bitcoin protocol in ways that safeguard against both known and unknown threats.