19. No, they've always run commercial operating systems
NSA really, REALLY liked VAXes - if you were building a new system and you had enough lead time to get the preferred hardware, your system would be based on a VAX. They also liked IBM 43xx mainframes and Series/1 minis. The standard desktop OS used to be Unix, but there was also a shitload of DOS in that place; from about 1983 on the most common desktop terminal was the Agency Standard Terminal Workstation, which was probably responsible for adding five years to the product life of the IBM XT because NSA bought thousands and thousands of XTs for this application. Right now the most common desktop OS is probably Windows. In applications where commercial off the shelf (or COTS) equipment will work, it's used because it's cheaper and quicker to implement it than to develop something in-house.
The flipside of this is, they are a government agency and as a government agency they are required to do the lowest-bidder thing, so they've got a lot of weird shit. Because of this lowest-bidder requirement, they used to be the dumping ground for things that weren't worth buying...William Odom, who was NSA director when Reagan was in power, once said that if we wanted to end the Cold War in one week all we'd need to do is airdrop half NSA's computers on the Soviet Union; they would try using them, decide we were a lost cause, and surrender out of the goodness of their hearts.