koobas.hobune.stream
"Bone Editor" does not appear in Sprite Editor's dropdown menu
...and Aunt Google doesn't want to cooperate on answering this one as well..
You have to install 2DAnimation package from Unity preview package . Simply go > Window >PackageManager Then Change to AllPackages and then Advanced > Show preview packages finally seach for 2D Animation then install it.
I see in my version of Unity 2019.6.4 it is called "Skinning Editor": 
