This has been posted here or in the other topic about the Bitcoin puzzle challenge. You don't like searching, do you?

I can only estimate and maybe RetiredCoder or someone else wrote something about it, but likely all unclaimed puzzles below ~100 have to be withdrown by a solution finder non-publicly. Otherwise the finder risks a "RBF war" and loosing the puzzle price either to some bot and/or being "converted" to transaction fee (you know what I'm talking about if you know how Full-RBF works).

To my knowledge the only public service to mine a transaction non-publicly is slipstream.mara.com.

We will yet have to see, if slipstream.mara.com can be trusted to handle a low entropy private key puzzle withdrawal transaction confidentially.

I tend to say that the solution finder for puzzle #67 would be stupid to publish his withdrawal transaction in public mempool.
The finder should document properly that he knows the public key because he found the private key at a certain point in time. Hash this message with SHA-256 or SHA-512, make the message hash public and submit his transaction to slipstream.mara.com.

I don't know if a finder could find a safe agreement with a larger mining pool offering a substantial transaction fee and having his transaction mined non-publicly by such a larger mining pool without risking being ripped off. May only work if he has a message hash published that documents his agreement with the larger mining pool. But this is not fool-proof, obviously.