It didn't die, it evolved. Mojang began taking cues from the modding communities, improving the internal APIs, making the overall codebase more generalized and

It didn't die, it evolved. Mojang began taking cues from the modding communities, improving the internal APIs, making the overall codebase more generalized and robust, moving model data to declarative JSON, adding registry systems instead of hard coding, introducing additional resources namespaces by default, resource packs, data packs, simpler creation of custom blocks, biomes, and most recently opening up Mojang's own class and method name mappings so that the insane process of mapping obfuscated Java class and method names can be stopped, allowing mod frameworks like Forge et al to have consistent API making and to reduce the previously huge amount of time needed for mods to get ported to new versions. The biggest problem with the "mod API" is that, effectively, you'll need to open source Minecraft entirely to satisfy the kinds of changes mods make to the vanilla experience, or else everyone will keep using the patch systems we are building on now. It is discouraged unless absolutely needed, but mods can even apply bytecode level runtime patches on the vanilla classpath to enable functionality that the mod framework hasn't yet exposed.