Mantle, announced by AMD in 2013, is a low-level API that provides developers direct access to its Graphics Core Next [GCN] line of GPUs, in 2013. What that means, essentially, is that developers can squeeze significant performance increases out of graphics hardware by not having to deal with the overhead of DirectX. The idea was interesting enough that John Carmack said, shortly after it launched, that Mantle could give enough of a boost to Steamboxes to make Microsoft and Sony "downright hostile" toward them.
But for whatever reason, it hasn't panned out, and now DirectX 12 is looming. Mantle offered great performance in a few games, but hasn't offered high-profile benefits over DX11 since the release of Battlefield 4 in 2013. Hardware site AnandTech said in its review of the AMD Radeon R9 Fury X that "AMD has now thrown all of their weight beyond Vulkan and DirectX 12, telling developers that future games should use those APIs and not Mantle." AMD isn't killing off Mantle, to be clear about it, but it is moving on to other things.
"In discussing the performance results of the R9 Fury X with Mantle, AMD has confirmed that while they are not outright dropping Mantle support, they have ceased all further Mantle optimization. Of particular note, the Mantle driver has not been optimized at all for GCN 1.2, which includes not just R9 Fury X, but R9 285, R9 380, and the Carrizo APU as well," the site explained. "Mantle titles will probably still work on these products—and for the record we can’t get Civilization: Beyond Earth to play nicely with the R9 285 via Mantle—but performance is another matter. Mantle is essentially deprecated at this point, and while AMD isn’t going out of their way to break backwards compatibility they aren’t going to put resources into helping it either. The experiment that is Mantle has come to an end."
It wasn't a complete bust, as Mantle helped shape Vulkan, a new, presumably better low-level API announced last year as the "Next Generation OpenGL Initiative." But the downside for gamers is that Mantle-optimized games—of which there are, fortunately, relatively few—won't enjoy those optimizations on future AMD graphics cards. Mantle-optimized games will still be playable (unlike my ATI 3D Rage edition of Mechwarrior 2, thank you very much) but the performance won't be anywhere near what it was when Mantle drivers were actively in development.