What happen to the 50-cent coins? Have you seen one recently?

I think the last time I saw one might have been in the 1980's.

Per Wiki:

"The half dollar's circulation, aside from use in some casinos and movie theaters, has declined significantly. The value of silver had risen by the early 1960s to the point that it became worthwhile to melt down U.S. coins for their bullion value. U.S. Silver coins (those of 10-cent value and above, which contained 90 percent silver through 1964) began to disappear from circulation, leading the United States to change to layered composition coins made of a copper core laminated between two cupro-nickel outer faces for the 1965 - present coinage years. The Kennedy half-dollar design, however, continued to be minted in a 40 percent silver-clad composition from 1965–1970.

Initially the Kennedy halves were hoarded for sentimental reasons and because they were recognized as the only precious metal U.S. coin remaining in circulation. By the time mintage figures could match normal demand and the coin's composition was changed to match the newer dimes and quarters in 1971, both businesses and the public had adapted to a world in which the half dollar did not generally circulate. Other uses had been found for the half-dollar section of the cash drawer. People had gotten used to depending on quarters as the major component of change.

Most coins enter circulation through the change drawers of businesses. Hardly any businesses stock their change drawers with half dollars or dollar coins, and many banks do not stock these coins or hand them out as normal business practice, so the coins do not see much circulation.

Most vending machines in the United States do not accept half dollars which further curtails its circulation."

Got one last month !

If you really want some, you can get them from the U.S. Mint.... www.usmint.gov, but you'll have to pay a premium.