The feature has been fully developed and will be added to the next version of Blender.
Blender Developer Jacques Lucke has showcased a cool new feature that promises to make the lives of Procedural Artists much easier and will be added to Blender 3.4, an upcoming updated version of the software set to be released on December 7, 2022. According to a new post made by the developer, the previously-shown Viewport Preview feature for the Geometry Nodes toolset has been finalized and will certainly be added with the next big update.
In case you missed it, the Viewport Preview feature adds support for showing geometry passed to the Viewer in the 3D viewport, instead of just in the spreadsheet. The developer comments that the "viewer geometry" bypasses the group output, so it is not necessary to change the final output of the node group to be able to see the intermediate geometry.