Another thing to keep in mind, in my opinion obviously, is that an API can come in many different shapes and forms. Although it's definitely not directly comparable I do think it's fair to state that the current developments in 1.12 and 1.13 also seriously account for all this. After all: Minecraft has never been so customizable as it is now, I'm of course in specific referring to everything you can do with command blocks (which are now seriously enhanced through functions) but also to things such as customizable loot tables, advancements and soon crafting recipes.
It's definitely not the same as using Java to program plugins for Minecraft, but it does come a long way as well.
On that note: I don't recall a specific source for this (possibly one of slicedlime's livestreams,) but the word I heard is that the plug-in API for PE (possibly Java?) will be based on C#. Also, there are going to be limitations, although I don't recall hearing anything specific, only that you would not be able to do everything you could do with true modding.
This is kinda old post so is there anything new about it
Several YouTube Minecrafters went to Mojang for a Creators Summit this past month, including docm77. In his vlog for Day 2 (near the end, don't remember time stamp,) Grum tells him the modding API (for Java) should be done around Christmas.
Another YouTube Minecrafter, slicedlime, was hired by Mojang this year. In several of his livestreams, where he answers some questions people ask in chat while he's doing some challenge, he stated he was hired to work on the modding API for Bedrock (MCPE.) He hasn't mentioned any due date, though.