Yeah it's weird that they did nothing for so many years. Maybe some of these old providers are a skeleton of their former selves? But they still have new games coming out.
Rival seemed to have a habit of quietly nerfing some of my favorite games without a word, so honestly the main games I miss from there at this point will probably be the progressive jackpot slots which I liked to take a shot at sometimes. It's kind of hard to trust a provider when your favorite games secretly become virtually unwinnable.
Two of my favorite were "i-slots" that had a skill based feature, cosmic quest 2: mystery planets and rock on. They were both fairly generous RTP slots because you lose a lot of that RTP back if you make a mistake in the feature mini game. All of a sudden sometime last year I simply stopped winning on those slots, particularly cosmic quest. Since it was a fairly high variance game, it took a while to be certain they nuked it, but I also noticed that the RTP which used to be listed at some providers was removed entirely from the help page. Similarly, rock on had some odd pay table changes, I think the top symbol was cut into half of its value but they randomly added a bigger possible feature win for 4 and 5 scatter triggers. I suspect they cut the wild frequency slightly in both of those games, and since the biggest wins in both games involved hitting expanding wilds on multiple reels, that took most of the big wins entirely off the pay table.
Well, now those games don't exist anymore and I wouldn't play them if they did, I wasted way too many busted game play sessions on them last year before coming to my senses.
Not sure if any games are missing but bovada seems to have converted most of their old RTG games, but they look a bit like cheap knockoffs of their former selves. If they were going through the effort of retooling the games, the least they could do is let us set the autoplayer for higher than 25 spins? It's a bit of a pain to reset now because you have to go to the menu, click spin settings, click like 5 more times to get it to 25 spins (maybe more since clicks don't register sometimes), exit the menu, then click spin every time the autospin completes. You used to be able to get the old versions to glitch so they would keep spinning as long as you wanted, don't think that's possible now though. But at least the games still exist. (Edit: did aztec's treasure used to have music in the free spins? Can't remember for sure but they might have lost some of the game specific sound/music as well. But not 100% sure on this. Edit 2: I guess the music might be there but it stops playing when you actually start spinning the feature? Weird. They also seem to have redone all the art except made it worse instead of just copying the assets over.)