Unreal Engine 4.15 is making OpenGL ES 3.1 / Metal / Vulkan editor feature level no longer experimental, but their Vulkan efforts appear to be mostly focused on mobile / Android support with not seeing any changes specifically for Vulkan on Linux/Windows. Vulkan work includes an updated glslang, texture streaming support on Vulkan, support for the global clip plane on Vulkan, and more.
Unreal Engine 4.15 also has experimental content hot-reloading, Linux ARM64/AArch64 support, finalized work on texture streaming optimizations, extended support for Apple's Metal API, new nodes have been added to the material graph, and a wide variety of other updates. There is also vehicle support being punted off to a plugin, AFR SLI support, font improvements, pose snapshotting, editor improvements, and more.
Aside from the Linux AArch64 support there is also improved Linux build support, install script fixes, CEF support on Linux x86_64, support for launching a project remotely on a Linux system via the editor/UFE, library improvements, and a range of other updates and fixes.
More details on Unreal Engine 4.15 via this blog post.