That's something I note toward the end. Although they're reappearing in Let's Go, they'll probably not appear in future games in any sense, except as a tacked on feature toward the end of the game. I included it, as a gimmick, because it seems like it was introduced in generation VI, and probably won't be expanded upon in any sense past Omega Ruby/Alpha Sapphire. Although it doesn't fall in the same vein as the radio or a lot of other features, since we don't yet know what it's future will be, and it's still being glanced over in most games, I felt it belonged on the list. If Pokemon ever needed to trim the fat, so to speak, I wouldn't be surprised if they decided not to move over mega stones at all.

As for Unown, there are quite a number of Pokemon I could have listed, but Unown seemed like one of those gimmick Pokemon with a lot built around it, only to ultimately fall flat. You could argue Pokemon like Minun and Plusle, or Chatot, or even Milotic, are also gimmicky. I could have included any of these, but Unown seemed more memorable.

And sure, HMs are not around in a formal sense, but I'd argue they're still very much around, even if not in their original form. Zones are still built around surfing/cutting/strength/rock smash - they allow for continued linear gameplay, restricting players from entering areas too early. The difference is that you don't have to have a Pokemon on your team that knows the move anymore.

The other HMs have also seen use in every game before generation VII. These mechanics were fundamental aspects of the Pokemon experience, and not tacked-on, experimental features.