So, uhm...apparently...
It's confirmed that we killed the Physical forms of C'thun and Yogg'Saron, despite their echo's still existing across Azeroth. However, that doesn't mean we won't see manifestations of them in the future (Y'shaarj had this case with the Sha).
Source: The Blizzard Interview for r/WoW.
Question: Can you confirm or deny whether the old gods are dead? We've got a lot of people who think that we've only fought manifestations, and their true forms lie dormant somewhere. Are we maybe going to see more of that?
Answering: Steve Danuser
Confirms that we've killed their forms, in the cases of Yogg'Saron and C'thun despite echoes of them still seeming to permeate the world.
"If you think about our cosmology and the way that creatures of magic work as opposed to mortals, mortals die they go to the Shadowlands. If you fought the Legion, you fought demons. If you kill them on Azeroth, where do they go? Back to the Twisting Nether, which is the place where they come from."
"So if you think about other magical creatures and think what happens when you kill them on Azeroth, where do they go? There's the potential for things like that to kind of happen. We try to have this cosmology of the way things work, and that's something that you can apply to other things. And I think the old gods are an interesting case where, you know, we've defeated one version of them and who knows if another manifestation will eventually take place."
https://www.reddit.com/r/wow/comment...e_danuser_and/
Basically, I also called it, and so does 8.3's datamining BS. N'zoth was the last Old God, and was the only freed one post-Titans.