Gas hypersensitisation, as you may know, is a method which was regularly used by astrophotographers. It included baking or soaking a film in a gas mixture, with this gas oftentimes being nitrogen.
As far as my understanding goes, gas hypersensitisation increases the film's effective sensitivity by diminishing reciprocity failure, while simultaneously decreasing film grain.
As film photography has started to die off to make room for digital photography starting from the turn of the century, the demand for hypersensitised film has apparently significantly died down as well, since I am unable to find any of such films for sale.
This person appears to have set up his own hypersensitisation apparatus, but this seems to be for personal use only.
I wonder if there are still companies producing and selling gas-hypersensitised films, especially in (western) Europe. I am additionally interested in how much such films would cost, and how much they used to cost in the prime time of film photography.