Homo sapiens interbred with them and their genetics still live on in the local populations. In fact this is a major reason how different human populations around the world became so different looking in such a short time.
"According to the Canadian anthropologist Peter Frost, the current level of hair colour diversity in Europe would have taken 850,000 years to develop, while Homo sapiens has been in Europe no longer than 45,000 years. This is evidence enough that genes for fair hair were inherited from interbreeding with Neanderthals."
Genes from Dennisovans, which had been living in Tibet for a very long time, are also why modern Tibetans are better adapted to high altitudes than other homo sapiens groups living in similar conditions.
Homo sapiens interbred with them and their genetics still live on in the local populations. In fact this is a major reason how different human populations around the world became so different looking in such a short time.