We know that archaic humans of some sort lived in India before anatomically modern humans (AMH) arrived, probably for more than a million years. But we know little about them, because we don’t have the required fossils. Plenty of stone tools have been found, but the fossil record is very meager.
India should have been a relatively friendly environment during the Ice Age, at least compared to Europe, most of which was uninhabitable at the glacial maximum. It could have sustained relatively large populations (except after Toba, perhaps). This seems to have been the case after occupation by AMH: mtDNA diversity suggests fairly large populations during the last ice age.
So who were these archaics? Might have been Neaderthals. Maybe Denisovans. Or, maybe, there was a hominid lineage there that had been largely isolated for a long time. Indian is somewhat geographically isolated today, but the degree of past isolation depends on past climate patterns. I don’t know what the Thar Desert was like 20,000 years ago.
If India did have a significantly isolated archaic population, it would not surprise me if we found evidence of admixture from that source in modern people. There have been some calculations that suggest more archaic admixture in Eurasians than is explained by known sources like Neanderthals and Denisovans – India might be part of that story.