My friend uses the Blender 2.7 interface itself to host DnD games, where each player gets a different view, a different camera and different layers each, in multiple windows. That’s where this little button comes into play, disabling it allows you to have different layers and active cameras per 3d viewport
I can’t find it in 2.8, but I assume the functionality is still there, considering that the option to set a Local Camera is still in the N sidebar, just greyed out as it would be if the option were turned on by default. Sadly, I’m not versed in the Blender API enough to toggle it with a command line and see if it works.
So either I’m blind and the button is there (my apologies if it is), or it was removed, hopefully temporarily, as the new collection system was still being implemented and things were still too green.
So, A) Is it there?
B) If it isn’t, please tell me it will come back
If that’s the case, I’ll make a design proposal in RightClickSelect about it
(P.S. Sadly Viewlayers don’t cut it. since they are per main window, not per 3d view, and do nothing about the cameras)