I've put up my first assets on the #Godot Asset Library - a script you can extend to make your CharacterBody3D support stairs!
GDScript: https://godotengine.org/asset-library/asset/2278
C#: https://godotengine.org/asset-library/asset/2279
You can see a quick test preview of the functionality in this post: https://mastodon.gamedev.place/@Andicraft/111256653707796501