The news comes via Anandtech's AMD Radeon R9 Fury X review, which notes 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."
While Mantle drivers will still exist for those with GPUs, AMD told the site it has "ceased all further Mantle optimization". There has been no Mantle driver optimisation for the whole GCN 1.2 range of AMD GPUs, for example. You’ll still be able to play Mantle titles on these cards, but performance will vary – probably in a negative way.
It’s a shame Mantle didn’t work out; the API gave developers unusually direct access to GPU tech, but it was only implemented in a few games before sort of vanishing. On the bright side, AMD is now focusing on DirectX 12 and Vulkan, both of which are cross-platform APIS, meaning everyone can benefit from them.