The hotkey / shortcut “H” always open/close the Heads Up display.
Changing the environment to show the backplate was always “B” (“C” for color and “E” for Environment).
In the render settings (options in dialog “Render”), you need to activate “Include Alpha (transparency)” (except available for JPG), to “hide” the background from the render results.
To get a backplate reflected on a glossy material, you need to use image pins in the HDRI editor, or create geometry with the image applied on a emissive or area light (both can be set to “not visible vor camera” and “visible in reflection”. This geometry can be hidden via shortcut (Ctrl + Alt + LMB by default).
KeyShot changed some shortcurts to implement another system in future releases.
Hope that answers your questions a bit.
CheerEO
Marco