Where it says “Axis and Action mappings are now deprecated, please use Enhanced Input Actions and Input Mapping Contexts instead” there’s a documentation question mark link to https://docs.unrealengine.com/5.1/en-US/Gameplay/Input , a 404.
ETA: the link should possibly go here instead: https://docs.unrealengine.com/5.1/en-US/enhanced-input-in-unreal-engine/