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.
- Nintendo 64 — Corn Kidz 64
- Game Boy Color — Mina the Hollower
- Game Boy Advance — Pipistrello and the Cursed Yoyo
- Commodore 64 — VVVVVV
- Mid-late 90s PC — Hypnospace Outlaw
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.