I did not know that. Fascinating.
Yes, as a mechanic of yesteryear specializing in Chevy small blocks, I recall removing that carbon from the combustion chambers on cylinder heads. One practically had to use a hammer and chisel for some of it whereas a rotary wire brush just seemed to polish it. Engine oils got dirty quite fast and seemed to thicken after a couple thousand miles.
When unleaded fuels came out I was amazed at how easy cylinder heads and piston tops were to clean.
We have a 97 Chevy Silverado with almost 200,000 miles on it. It uses no oil at all and the oil stays clean.