There are many guns that have been produced over the years, and many models are no longer in production. However, there are situations where it is not a problem that a gun is no longer made 
because there is a really large market for replacement parts. Some example would include the earlier versions of the Colt snake guns and the Browning Hi-Power. There are several others.
Recently, Honor Defense went out of business. They made great 9mm handguns designed for the concealed carry market. The problem is that they did not sell a large number of guns, so there is no third party market for key parts like extractors and springs. Magazines are also difficult to find.
Is it a problem that you can’t find replacement parts for repair? Well, yeah… duh! Speaking from first hand experience, it sucks to own a great gun when you can’t even get a replacement recoil spring for it.
The lesson? While it is tempting to buy a quality gun that is no longer in production, make sure you can get parts.