This looks to be about the same: Workspaces in grid (#1560) · Issues · GNOME / gnome-shell · GitLab. The 1st comment specifically mentioned that how Workspace Grid/Matrix extensions did it didn’t fit with the design. I think that still holds true.
At the time the Shell had vertical workspaces and an activities button on the panel. Currently the Shell has horizontal workspaces and the activities button has been replaced by a dynamic workspace indicator. The workspace indicator on the panel, the workspace thumbnails in the activities overview and the workspace previews in the app grid all use a horizontal layout. How would arbitrary sized grids look and work in a consistent way for these 3? I think it can’t work without breaking the current design.