For what it's worth, here's what I've done to mitigate the problem: In my inputs area, I have camera 1 followed by all its presents, then camera 2 followed by all it's presets, etc. All the presets for all the cameras are collapsed (i.e., they are right-clicked). They are meaningfully labeled so I know where they are pointing without having to see thumbnails of where. This makes for clean, compact, yet informative display. When a preset is in preview or output, the entire collapsed "strip" gets colored by vMix per the Settings for Display. It's now very obvious when a camera is in use (either as preview or output) or not in use.

We've taken it a step farther with BitFocus Companion. Row 1 is camera 1 with all its presets, row 2 is camera 2 with its presets, etc. Each button is programmed with colored instance feedback for when in preview and output. In this manner, Companion now mirrors the coloring we have in the vMix inputs area. It's totally obvious if a camera is in use either as preview or output (i.e., if there's any color anywhere in the row, then the camera is in use). Side note, we also use the Elgato Stream Deck.

+1