Role of introgression to define the percentage of modern and neanderthal genome

Introgression, also known as introgressive hybridization, in genetics is the transfer of genetic material from one species into the gene pool of another by the repeated backcrossing of an interspecific hybrid with one of its parent species. Introgression is a long-term process, even when artificial; it may take many hybrid generations before significant backcrossing occurs. This process is distinct from most forms of gene flow in that it occurs between two populations of different species, rather than two populations of the same species. Wikipedia (2021-04-12)

If mixing between modern humans and Neanderthals happened through a period of several thousands of years, introgression must be taken into account to calculate timelines.

In the Neanderthal DNA deserts, there is no presence of genetic information even after few generations of interbreeding, pointing to a very quick selection against Neanderthal DNA[@hajdinjak2021Initial Upper Palaeolithic humans in Europe had recent Neanderthal ancestry].