Oh Subaru purists will have a lot to say about this.

In a statement, Subaru just announced that they are not producing the WRX STI. The reason is fuel economy and emissions:

As the automotive marketplace continues to move towards electrification, Subaru is focused on how our future sports and performance cars should evolve to meet the needs of the changing marketplace and the regulations and requirements for greenhouse gasses (GHG), zero emissions vehicles (ZEV), and Corporate Average Fuel Economy (CAFÉ).

As part of that effort, Subaru Corporation is exploring opportunities for the next generation Subaru WRX STI, including electrification. In the meantime, a next generation internal combustion engine WRX STI will not be produced based upon the new WRX platform.

There are quite a few takeaways from those two paragraphs. The first of course is that emissions regulations means the STI is unfeasible. Under CAFE rules, an automaker that cannot meet a certain standard have to pay penalties; this is in the US, but that also happens to be Subaru's biggest market that accounts for a significant portion of STI sales. The lion's share, probably. 

So that means the WRX STI is axed for now because Subaru has to develop a new generation engine for it. The EJ series engine on the STI is actually one of the oldest that was still in production, as the engine traces its origins to the late eighties. The WRX already had the newer (and more efficient) FA series engine, but Subaru never put it in the STI for series production.

So Subaru wants to significantly improve their emissions footprint with an all-new WRX STI. It's too soon to tell, but we expect that to be in some kind of hybrid form; perhaps mild hybrid style, but with a bit more kick. Whichever electrification style they go for, it will be with a new generation internal combustion engine.

But it'll be sad for fans that the WRX and WRX STI won't be produced together. Those two have always been a power pair. You went for WRX if you wanted a bit more performance, and you had the WRX STI if you're ready for even more.

So if you're waiting for an all new WRX STI to go along with the WRX, you'll be waiting for quite a while.