Formal:
* Extinct (especially for species)
* Defunct (for organizations, businesses, etc.)
* Obsolete (for things that are outdated)
* Nonexistent
* Vanished
* Disappeared
* Passed away (for people)
Informal:
* Gone
* Dead (for organizations, businesses, etc.)
* Out of business
* No more
* History
* A thing of the past
The best word to use will depend on the context. For example, "extinct" would be the best choice for a species that no longer exists, while "defunct" would be better for a business that has closed down.