I don't know why some people are so salty bout a suggestion.
Looking behind in vehicles is one way to avoid accidents, I suggested this very feature 'yonks' ago and go a similar response, the fact that it would help when undocking in a busy station or as Meresiga pointed out for landing in canyons seems to have been ignored by many posters, those who's keyboard prowess it seems is head and shoulders above that of us mere mortals.
But looking behind you pretty useless for any ship ABOVE you. or to your sides, or even below you. It is only useful for looking behind you, so what would the use be for this very limited use case?
So lets take a look at your scenarios.
Scenario 1.
You sit in a station, facing the exit. you turn on the rear view camera, and what you mostly will see is the docking pad building. So no ships there, you releases the docking clamps and move up. and hits a Beluga, waiting above you for its time to exit the station. If only you had asked for a top mounted camera instead of a rear view camera.
Scenario 2.
Landing in a canyon, near a canyon wall. Here you already have the radar image of the ground, to help you, it will show the wall. so I doubt any rear view camera would add to much, and since this would basically be a flat 2D presentation, your depth perception would be severely limited. So you could see that there is something behind the ship, but with no reference it would be hard to tell how close or far away it is. And if you try this in ship with wide wings, then you are very unlikely to see the end of the wings, and thus these could cause problem that you did not see.
Now, there is actually already ingame function to will let you see above, under, left, right, any direction really, the external camera, you can even fly your ship in the external free camera mode. where you either "dock" the camera to follow your ship, or "dock the camera to the "ground", so you can see the landing spot and navigate the ship into it.
This is why I asked what the use for this would be. And sofar I have not got any real usable use cases explained.
One of the few use cases where a rear view camera could be helpful is landing a big ship, like Beluga, Cutter etc. Where you sit in the front and have no real sense of your orientation, and end up having your butt in a some 45 degree up, but even in this scenario, you already have most tools already there to help you. You have the landing hologram, sadly they did not update this once Horizon launched, as the planetary aid to orient your ship is vastly superior to the old one used on landing pads. And you also have the external camera. So this would not be adding much use in this case anyway, that existing mechanism have already provided.