Nintendo is replacing its multiplayer server system, NEX (used in most, if not all of their online games since 3DS/Wii U), with a new system called NPLN.
The Monster Hunter Rise demo was the very first game to use this in public.
Nintendo is preparing a big multiplayer overhaul, probably for games in development in 2020: every task currently taken by NEX is going to be switched over to NPLN. It's currently in a preview phase, and the Monster Hunter demo was a way to test how it worked under load.