Wasn’t this one of the early civilizations that were overthrown by the big horse warriors from the steppe?
Yep.
It was possibly the earliest literate civilization as well.
Depends who's asked. :^) When the Aryans entered India, they found the great Harappan cities already deserted apart from the dead, who were numerous, and described as short and dark haired. Of course now the reality of the Aryan invasion is denied for political reasons.
The Rig (or Rg) Veda refers to the city, but just once, and never again. The cities, rivers, and overall geography of the ancient Indian epics appear to refer to a different place, and archaeologist Viktor Sarianidi (spelled about five or six different ways depending on the article) dug up their original or rather earlier homeland in Central Asia.
There are, however, Aryan Invasion Deniers (AID). :^)
I think so.