That's true, but they did still serve roles in the story, even if very briefly. Ryuki's role in the games is so small you could actually do a full playthrough and not even encounter him.They make a lot of NPCs thay don't carry over. Ryuki isn't anything new. I mean look at almost all the Kalos NPCs. None of them showed up in Alola. There's also the poor Megapolisians who will probably never show up again because of the Ultra Beasts being dropped.
Besides Ryuki seems more Japan based then British based.
But Ryuki wasn't really connected to gyms there-he doesn't appear in the Kanto gym at all in SuMo, and only occasionally in USUM.Couldn't he just exist for the sake of exposure? If your first games happened to be Sun and Moon (and undoubtedly for someone, they were), then still having a gym in the game means that you're still familiar with the concept of gyms and aren't alienated from the rest of the fanbase in that regard.