I actually haven't seen the rear camera get covered and have done quite a few k's on unsealed roads. I have had mudflaps on the car since soon after new - if you don't, I wonder if they help? If you do...it's not that
I keep a microfiber cloth hanging on my home charger and give the rear camera a gentle wipe every night I charge. I also have one in the car for use when needed out and about (rarely needed). My experience has been that the rear camera on our Model Y stays a lot cleaner than the one on our 2013 Model S did. Aerodynamics?
To specify it's on my model 3.
I also suspect aerodynamics as I haven't seen a back of a car getting so dirty as soon as roads are wet (rain or snow).
I do have mudflaps at the front but not at the back. That may be something to look at. However even with snow clogs on rear wheels, it does get muddy as hell.
I also wipe the camera each time I pass near it, but I am not seeing myself going out of the car just before parking just to wipe the camera
I was maybe thinking of some hinge protection like on the iPhone near the camera that could help ?
And the phenomenon appeared very early in my possession of the car wondering if the hydrophobic coating from factory was applied at all ?