Are there any truly "forgotten" civilizations?

You should know, this was precisely the seed thought of the OP.

Wait, what? Perjorative to a people no longer around?

Their descendants are.

‘Anasazi’ is the Navajo word for the builders of the sites. It has now been shown that they are ancestral to the Pueblo who find the term ‘Anasazi’ offensive. The generally accepted term for those who built the Great Houses of Chaco Canyon and surrounds, and Mesa Verde is ‘Ancestral Pueblo’.