Are Coins Cleaned At All?

The default assumption should be that any century-old coin which still looks brand-new has been cleaned/"preserved" in some fashion in the past. This not the perfect truth - many have absolutely been held with sufficient care to naturally preserve them - but all the same a whole bunch have been artificially treated as well. If the look is that of a naturally-preserved coin, the market tolerates it. Very difficult to tell anyway.

There are certain specific cases where it's appropriate to do something about conserving a coin. PVC plasticizer "infections" must be arrested. Coins which have been taped should probably have the tape residue removed. Ancients. Dug coins. The reason for the "never clean coins" advice is that these are all case-by-case decisions, different for every coin and it takes a certain level of knowledge/judgment to determine a worthy candidate.

Or it is "market acceptable". There are plenty or older US coins that have been cleaned or have problems that were slabbed as "problem free" back in the years before they had the details slabs. The TPG's were much more forgiving on 18th and early 19th century material than they were on late 19th and 20th century coins.