Depends on your definition of “new” and “must-have”.
Grapple Dog 75% off — momentum-based grappling hook platformer with a very funky soundtrack and Nitrome-style 2D art (spoiler: the dev was a Nitrome artist)
Cobalt Core 25% off — deckbuilding in space! There’s an actual plot with an interesting cast of characters. The board game-style positioning mechanic was what sold me on the game.
Cassette Beasts 40% off — creature collector with an identity that isn’t just “indie Pokémon”. Cool worldbuilding, heartfelt story, grown-up mechanics, and imaginative monster designs that avoid the “animal + element” formula.
The 2015 Monster Summer Sale was particularly memorable for me thanks to the music. I downloaded it and still listen to it! Ultimately, though, I’m glad Steam doesn’t have tie-in minigames and flash deals anymore, since it was tedious to have to metagame them for the best value.
Personally, I consider “retro graphics” to specifically mean the graphics evoke the look of an old game system, as opposed to just having “lo-fi” art. So I’d say that Celeste has great pixel art, but it’s not retro graphics, since it doesn’t remind me of any old console’s look. In addition to the games others have mentioned, I’ll point out these ones.
Indie games using retro graphics is nothing new. The ratio of effort vs looks is pretty good. The solo dev of Cave Story (released 2004) said that was why he went for that aesthetic.
Steam input is absolutely baller. I use it extensively with my Steam Controller. For FPS games, I have gyro aim activate while the right pad is touched.