While not actually released yet and not due until later this year with Godot Engine 4.0, the Vulkan parts have now been merged into the main Godot project.
In a new blog post on the official site written by Godot's Project Manager, Rémi Verschelde, it goes over what this means. In short: it's all highly unstable but now it's in the main branch, they can continue pushing Vulkan forwards and updating all parts of Godot required for it.
We plan to rework a lot of Godot's internals (core) to allow fixing long-standing design issues and improving performance (including GDScript performance improvements). Moreover, our long-awaited port to C++14 will also happen now that the vulkan branch is merged into master, and many other codebase-wide changes were waiting for this: code style changes, Display/OS split, renaming of 3D nodes to unify our conventions, etc.Rémi Verschelde
Having the Vulkan code in a separate branch could only work for so long, before it would cause too many issues. Going forward, they're going to remove the GLES3 support and port GLES2 to the new RenderingDevice API as the backup for devices that don't support Vulkan. If you missed why they're using GLES2 instead of GLES3 that was explained a while ago—terrible driver support being the main issue.
They're also going to be doing some cleaning up, closing all current pull requests and asking the authors to make sure it's still needed and approved by their Godot Improvement Proposals staging area. Well, that's one way to clean up hundreds of lingering pull requests.
Learn more about Godot Engine on the official site. It's free to use to make games and applications, with no need to pay royalties to Godot like some other game engines require.