It's important to note that Quechua wasn't just one language, but a family of languages, with several distinct varieties spoken across the vast Inca Empire. The variety spoken by the Inca elite and considered the "official" language of the empire was Classical Quechua (also known as Cuzco Quechua, after the Inca capital).
Even today, Quechua is still spoken by millions of people in the Andes region of South America.