As you probably guessed, this was an April Fools joke, and sadly, we’re still waiting for the Plugin API.
“Workbench”, the Minecraft Plugin API that’s been in the works since July 2010, is finally here. Well, not quite.
We were contacted yesterday by a mod developer who’s been invited to test out the API and give feedback. The source asked not to be named, but provided screenshots and some details.

An API is a framework that allows developers to hook into a piece of software and build upon it. The Workbench API gives Minecraft developers a layer to hook into when writing their plugins, meaning that Minecraft, while still allowing compatibility across multiple mods. The API is also designed with multiplayer in mind, meaning that at the moment, singleplayer plugins are still an “in-development” feature.
Plugins are added in a way similar to Resource packs, although there are apparently plans to have plugins centralized on the Minecraft Website, with the ability to add them directly through the game. Whether or not this happens is still up in the air, as Mojang has to decide whether they want to jeapordise the good will of the mod community by possibly removing a source of revenue from many developers who use services like adf.ly.
We don’t know when the Mod API will be released to all developers, or when players will be able to install plugins, but it’s looking interesting so far.