someone doubled the prize money
Also, there is some traffic related to this address on testnet3.
I myself could reproduce the private key from scratch, using only the information that is publicly available on the blockchain.
I have made it so that the puzzle can be solved with any computer that could run Bitcoin Core.
I could be wrong, but if the whole challenge can be completed with Bitcoin Core, then I guess it is all about signing some message, because Bitcoin Core can sign any message with any key, there are commands for that. And then, it may require just grabbing R-value from the signature (that is deterministic), and using that as a public key, so the private key could be then reached by reproducing k-value for some trivial key with trivial message. But even then, there are a lot of options, definitely more than I could brute-force. So it is not yet taken, because it is based on "security through obscurity".
Another question is if that challenge will be upgraded to Taproot? Because if it is really based on HMAC-SHA256 or things like that, then it could be as well based on Taproot, where there are endless possibilities for a brainwallet TapScript, exactly in the same way, as there are endless possibilities, when it comes to signing some transaction. As I said, it is based on obscurity, and that's the only reason why nobody moved it yet.