If I buy an iPhone 8 from USA with AppleCare+ warranty plan, then would the warranty hold in India?
Be careful but also you might be fine in many / most cases. Also, AppleCare and AppleCare Plus are not warranties but service programs.
Warranty is certainly one very good form of legal consumer protection. This body of law affects and extends and restricts business and you only get this protection from the country of purchase and generally for less often from the country of import to.
Just as with warranty, there is language that allows Apple to restrict service to the country of sale:
Apple may restrict Hardware Service and ADH Services to the country where the Covered Equipment was originally purchased.