How to create Sloped Walls in Section

As of Revit 2021, it is now possible to create Sloped/Slanted Walls.

Adding a Sloped Wall

You can place a sloped wall in any view that you can model a wall. For example, Floor/Ceiling Plan, Elevation, Section, 3D orthographic or Perspective View. However, a Floor Plan is the most accurate and easiest.
Select your desired Wall from the Type Selector on the Properties Palette
You can set the Slope before or after placement, to do so before, on the Properties Palette ► Constraints ► Cross-Section, select Slanted. This will enable the Angle From Vertical parameter to display.
Using the Angle From Vertical parameter, specify the wall’s angle between +90º and -90º where 0º is vertical.
You can also change a vertical wall to a slanted wall using this parameter. If the wall contains inserts (windows, doors), sweeps, or reveals, they will remain in a vertical orientation, and you must adjust their orientation manually using the Orientation Instance property.
Use the in-canvas dot controls at the top of the slanted wall to adjust the wall angle and top offset. Snapping behaviour is enabled by default, and guidelines display to allow greater accuracy.

Sloped Wall Properties

In-Canvas Editing with Grips

NOTE:

Positive or negative slant, as indicated in the Angle from Vertical parameter, is determined by the direction in which the wall is drawn. In the draw direction, right is always positive and left is always negative.