https://github.com/Universal-Team/pkmn-chest/releasesWas of course the first thing I did, but I only
found Pokémon Cards.
But on the same site I got PKSM I found PKMN-NTR. Could be this app. Bur sadly only for Gen 6 and 7.
By the way, can the forwarders on the 3ds be outdated somehiw and not work anymore in the future?