It is, but it cost over $20Is subnautica not from an indie studio?
Kenshi
They Bleed Pixels. Happens to be 1.99 right now on steam, normally $10. Love the atmosphere of this one.
Touhou Luna Nights, a metroidvania, feels very polished.
Plague Inc.
We somehow forgot to mention Mount & Blade
Two free down the rabbit hole options:
Cataclysm DDA: ascii (or tiles) zombie horror survival RPG with extensive character customization and neat mechanics. Has a nice sci-fi spin to the genre. It's very similar to Fallout 1/2 in ways. My last character was a fireaxe wielding drug-addict cyborg that had schizophrenic hallucinations and eventually died to a grenade launcher wielding zombie. Project Zomboid is based off of this game if I'm not mistaken.
And Dwarf Fortress of course. It's Rimworld on crack and Adderall. Simulate a world's entire history and then join in. Be sure to dig too greedily and too deep.
I think I have 250 hrs in Kenshi. I hope they add more map and lore and take more of my money. I kinda want a Bone Dog and Pack Bull to follow me everywhere in RL.
Child of Light may not be an indie game technically since it's published by Ubisoft oddly enough. But it certainly fits what you would expect from one. It's also 4.49 on ps store right now and is really good.
Cosmic Star heroine is the only Indie game i have that i don't see mentioned so far. It trys very hard to be a 16 bit rpg so if you liked those SNES rpgs you might wanna give it a try.