To give you a concise answer, USA Federal law only prohibits discrimination based on the following characteristics:

Race. Color. Religion or creed. National origin or ancestry. Sex. Age. Physical or mental disability. Veteran status. Genetic information. Citizenship.

Anything else you can think of is a legal reason to discriminate

you could argue that caste, is a combination of color+religion+origin/ancestry combo... it has a racial + religion + ancestry undertone.... i.e. it is something you are 'born into'.

Sure. But maybe to complex for most people. And more often than not, people seem to be quite happy with excuses to not do anything.

I would say caste is ancestry based discrimination

But not national ancestry, which is what is protected.

I remember HR training circa 2010 specifically taught me that I could discriminate individuals based on their Sign of the Zodiac as that was not a protected area.

Moreso, it's not yet illegal because the problem is an imported one entirely.