Skew? Is It available?

Sometime i see skew option in Gizmo properties window but i dont know how to turn in on. Any advise how to do it ?

You need to rotate the pivot diagonally and then use the gizmo to scale the object diagonally. Then scew appers.

Hidden by default as I want to discourage skew and non uniform scale.

But otherwise yes, skew appears in the interface if the matrix has it.
The simplest is just to rotate the pivot and the use one of the non uniform scale gizmo handlers.