"They" have not stopped making 20 cent coins.
In 1964, the US mint was making coins primarily at two sites, Philadelphia and Denver. To distinguish the coins, the Denver coins had a D under the date. The Philadelphia coins had no mint mark.
Your question needs to be rephrased. "Denver coins" are simply ordinary US coins (cents, nickels, dollars, etc.) that were struck at the US Mint facility in Denver.
They stopped because making the coin costed too much.
Indian head (a/k/s buffalo) nickels were made from 1913 to 1938. 1938 coins were only made a Denver, as a temporary measure until the new Jefferson design was ready.
1867
D coins come from Denver, Co
Banks in Texas tend to get most of their coins through the Denver side of the supply system.
The world wide increase in silver prices was the main reason.
No, the Denver mint made these coins on purpose.
Current circulating U.S. coins are minted in Philadelphia and Denver, with collector proof coins minted in San Francisco.
Denver. That's why some coins have a little "D" mintmark.