Unreal Engine 4.16 was released today and with it brings full functionality for the Nintendo Switch platform. Although the engine has technically allowed for Switch development since its last official update in February, it was only classed as experimental. Now that the engine has been updated, approved developers can download the UE4 source code for free. Coming from their blog:
Support for Nintendo Switch is fully-featured and ready for production in 4.16! Epic Games has teamed up with Nintendo to release the full UE4 source code for Nintendo Switch to approved developers for free.