Thanks palomnik, but I have a question about your answer. According to the dictiionary, "extinct" can also mean:if a type of person, custom, skill etc is extinct, it does not exist in society any more. Why is that?
I think we don't say "extinct" too much for skills because that means completely gone. No human being uses it anymore. Maybe "becoming extinct" or "die out," because it's not completely out yet, but it's fading away.
When animals or anything become extinct, you will never see them again.
Extinct is most often used when referring to species of animals disappearing from the planet. When referring to animals, "die out" or "extinct" will work.
The dinosaurs went extinct. The dinosaurs died out.
Other things usually have other terms. Perhaps because few other things really go extinct.
Many aspects of our daily life are cyclical over the years. Bell bottoms were in, then out, then in (briefly) and then back out again. Typewriters, while rarely used anymore, have taken on antique status and have become valuable to people in other ways. Hair styles come and go and come back again. Many things do, which is why I believe extinct is most often reserved for living creatures.
Yes, I'd agree that "die out" would be by far the more common term to use for skills, customs, practices and ideas. As evilregis observes, sometimes old customs don't really die, and perhaps a better synonym is "fallen into disuse."