Can the fees in a coinbase be used to launder Bitcoins?
While nothing stops you from paying a high fee to a miner and claiming it out of band, trying to launder any significant amounts through this would likely be discovered and linked fairly quickly.

A pattern of disproportionately high fee transactions appearing in blocks that are mined by a subset of miners would be noticeable, as would linking the money - If you are laundering 10 BTC and put it through via these fees, I simply need to track the spends of the coinbase output instead of spends from the original 10 BTC utxo.

Of course, you could do this piecemeal by laundering smaller, less noticeable amounts like 0.01 BTC per tx, up to 0.2 BTC per block, but that doesn't really scale to a significant amount, especially not once you take out the miner's fee. Moreover, unless you are able to bring all miners on board, you could only do this a handful of times a day (assuming you manage to convince a single large pool), which again does not scale up to any significant amount of BTC, especially if you stick to doing small amounts.

It's likely simply not worth the effort for miners or launderers. Sending coins to a KYC free exchange, or exchanging them P2P for other coins, especially ZCash and Monero, likely has much higher throughput.