There was a thread about this transaction made at the time, where I discussed what has likely happened here:
Think twice before pressing send. It is almost certainly a mistake from someone who really should have been testing whatever they were testing on testnet.
The transaction does not have any change. Just one input of 3.49 BTC and one output of 5000 sats.
Usually this happens when someone tries to create a transaction manually or is programming some new piece of software. They set up a small "test" transaction of a few thousands sats, forgetting that you must specify a destination for the full amount of bitcoin you are spending, and the fee is simply calculated by whatever fraction is left that you have not assigned a destination to. Since they only assigned a destination for 5000 sats, the remaining 3.49 BTC was used as a fee, working out to 1.8 million sats/vbyte.
BTC.com mined the block, so the user in question should contact them, explain the situation, sign a message from the input address, and ask for the majority of the funds to be returned. Other mining pools have done this in the past when such obvious mistakes have been made.
I have seen nothing from BTC.com or crypto news sites in the last few months to suggest that the sender has attempted to recover this fee, however.
No, the error is from the user, no transaction is untreceable as the bitcoin blockchain is transparent.
Traceable, sure, but it could still be used to launder money. I pay a fee of 3 BTC for a transaction, include the transaction in my own block, then claim the fee as a nice, clean "mining reward".