Does anyone still need testnet3?

A novel proposal suggests creating an application to enable coin swapping between testnet3 and testnet4, potentially incorporating an Automated Market Maker (AMM) system to facilitate this process. This initiative aims to simplify resource acquisition for developers, fostering a more efficient testing environment within the Bitcoin ecosystem.

Testnet4 is currently experiencing frequent short reorganizations (reorgs), primarily due to the exploitation of a rule that allows mining difficulty to drop significantly after a period without new blocks. This scenario leads to a situation where the Median Time Past (MTP) on testnet4 is often set in the future, an anomaly diverging from the expected behavior. In response, a countermeasure involving strategic re-organization of blocks mined at artificially low difficulty has been proposed. Though this solution is still conceptual and not intended for direct incorporation into Bitcoin Core, it offers a potential pathway to increase the frequency of reorgs, serving as a valuable testbed for debugging and further exploration.

The integration of testnet4 within the Bitcoinj framework is underway, though not yet complete, with the next release (version 0.18) anticipated to take longer than 12 months. Despite these timelines, the current version of testnet3 remains a satisfactory testing ground, even with its challenges, which are deemed beneficial for stress-testing code. The commitment to support both testnets moving forward was confirmed, alongside a mention of btc-rpc-explorer, a self-hostable block explorer tool, which currently does not support testnet4. These developments highlight ongoing efforts to improve testing frameworks and tools, reflecting the community's dedication to enhancing the robustness of Bitcoin-related technologies.

Testnet4, proposed in BIP94 and supported by Bitcoin Core from version 28, addresses the limitations of using testnet3, including block storms that have made it increasingly impractical. This transition allows users to either adopt testnet4 or continue utilizing testnet3 by maintaining their node software independently, underscoring the blockchain community's commitment to backward compatibility and adaptability. The discussion also opens the possibility of creating a permissionless signet based purely on proof-of-work, indicating an innovative approach to improving network functionality. Feedback from developers reliant on testnet3 is encouraged to ensure that future versions of Bitcoin Core, such as v29, remain inclusive and accommodating, highlighting the importance of community input in guiding Bitcoin's technological evolution. Interested parties are invited to contribute to the dialogue through the provided Github issue link, facilitating a comprehensive discourse on these advancements.