If you assign a custom key command (“R” in my case) to the following function under the ‘Mixer’ folder:
Show/Hide Channel Racks
… does using that key to show/hide the channel racks in your mixer windows work? This works exactly as expected in Nuendo 8, but I’ve had no luck with it in Nuendo 10. Selecting/Deselecting the checkbox in the mixer window’s configuration does work, however … just not the assigned hotkey.