I appreciate now that setting a 2x2 grid of desktops is now feasible…
Plasma 6.2 now arranges them in a row (using arrows to navigate, we see also that the first two are the top two, the second two is the second row.
Using the shortcut Cycle through Overview and Grid (Meta_Z on my system) then brings the grid.
The regression is that the OSD/overlay does not work when switching desktops.