“Open-source” is not up for interpretation. The word was coined by this definition being made public: https://opensource.org/osd/

For anyone left behind, Minetest is a community-developed alternative.

It’s more of a game engine/launcher + highly moddable, so the base game is rather minimalistic, but you can simply install more extensive games. For example, for a very Minecraft-like experience, MineClone2 is your best bet.

My personal favourite is Dungeon Crawl Stone Soup, although it’s quite tricky to get into and still brutally difficult, even if you are in it.

It’s similar in concept to Shattered Pixel Dungeon, which others have mentioned here. Maybe try that one first. 🙃