Does anyone still need testnet3?

The testnet4 environment is currently experiencing frequent short reorganizations (reorgs), primarily due to individuals exploiting a specific rule. This rule allows the mining difficulty to drop to 1 after a 20-minute period without a new block being mined. As a result, for every block mined at the actual difficulty level, miners are able to quickly publish five to six blocks by leveraging this difficulty drop, adhering closely to another rule that prevents blocks from being timestamped more than two hours into the future.

This behavior has led to the Median Time Past (MTP) on testnet4 typically being set in the future, diverging from the usual pattern of it being about an hour in the past. This anomaly, alongside the exploitation of the difficulty adjustment mechanism, has inadvertently assisted in uncovering several bugs within the system, thanks to the implementation of a timewarp attack mitigation rule.

In response to these issues, a potential countermeasure has been proposed. This solution involves strategically re-organizing those blocks that were mined at the artificially low difficulty. However, this countermeasure is currently conceptualized only as a hackathon proof-of-concept and isn't expected to be integrated into Bitcoin Core directly. Implementing this strategy could potentially lead to an increase in the frequency of reorgs, providing an enhanced testing ground for further exploration and debugging.

For further details, references [0], [1], [2], [3] provide additional insights into the issue, while reference [4] offers a deeper dive into the proposed countermeasure.