By 2500 BCE, what did the Harappan civilization develop that would not be seen again for 3,000 years?

Indoor bathrooms

Explanation:

Based on the sites that left over by this civilization, Archologies noticed that   there are rooms that were most likely used for indoor bath.  Not all building continued these bathrooms. Archeologists believed that it's only common among the upper class members of Harrapan civilization.

Judging from the size of the bathrooms, it is unlikely that they're used for personal bath. It's more suitable for some sort of group setting like for courtesans to entertain a noble. This type of bathrooms did not exist in other civilizations in that era. The closest ones found in Roman empire in around 390 AD.