The state of open source Minecraft is nuanced enough to be interesting. The Minecraft ip and codebases are currently owned by Microsoft and have technically always been closed source, but the nature of the game and its development history are such that third party mods and modding apis are very common. Because the original edition of the game is written in Java, it's relatively easy to decompile and develop for. In fact, Mojang Studios has been including a deobfuscation map with each snapshot release of the game for a few years now which would allow you to decompile the Java edition of the game and use the same names that they use internally [0]. Though, most modders prefer to use the mappings provided by modding apis like forge and fabric due to tradition and licensing conflicts. Depending on your definition of Minecraft, there's also plenty of open source implementations of Minecraft Classic, a much older version of the game that's easy to reimplement but apparently still has an active community [1]. Then of course there's Minetest, an independent totally open source voxel game that happens to have a lot in common with Minecraft [2].

I should probably also note that there's something of a conflict in this area surrounding the other edition of Minecraft, Bedrock Edition, which is written in C++ and therefore difficult to decompile and mod. Bedrock Edition is much more closed than Java edition in a number of ways, such as including a store built into the launcher for buying maps, texture packs and skins whereas these would need to retrieved externally (usually for free) on Java edition. However, Bedrock Edition does have a first party modding api, a more feature rich internal scripting system with the concept of "behavior packs" [3], and has led to the development of a number of tools used by mod authors for both editions, such as Blockbench [4]. Ultimately Java Edition and its community have the legacy of Notch and Mojang with a long history of community contribution while Bedrock Edition was only developed after the Microsoft acquisition and is much more in Microsoft's style.

EDIT: Turns out the Bedrock Edition modding api was discontinued earlier this year [5].

[0]https://minecraft.fandom.com/wiki/Obfuscation_map [1]https://wiki.vg/Category:Minecraft_Classic [2]https://www.minetest.net/ [3]https://learn.microsoft.com/en-us/minecraft/creator/document... [4]https://www.blockbench.net/ [5]https://www.minecraft.net/en-us/creator/article/removing-the...