Hi and thanks for you interest in Halcyon.
The whole project was created in Unreal Engine 4. The islands level (the one all the screenshots are from) uses very simple techniques. Models are low-poly with each polygon's normals set to 90 degrees to create hard edges. Shaders in this level are mostly very simple, no supporting maps or textures - everything defined in UE4. The exception is the grass, which uses an alpha-masked texture with a slight gradient to it. The mask was painted in Photoshop with individual brushstrokes, to give the grass a less realistic and more painterly look. UE4 has some great post-processing effects which really help to tie the scene together.
Other areas of the game use more complex shaders with detailed diffuse, roughness and normal maps to create a more realistic look.
If you like my work, I'm currently working on asset packs to release on itch.io at some point in the future (I don't get much time to work on that stuff).
Hope I answered your question and thanks again for the interest. :)