Can I get a confirmation about Live View and the Histogram in low light situations, like Milky Way photography? I find that at some point the histogram stops changing to reflect the changes in shutter speed or ISO. The histogram will not move above about 1/4 up from pure black, even if shutter times are extended or ISO is increased significantly. Once this limit is hit I must resort to chimping; taking a picture and checking the playback histogram. I can't find anything in the documentation or online regarding this.
I have this experience on my 5D4, 5D3 & 6D
What you are experiencing is Live View exposure simulation failure and that is normal when the scene becomes too dark, like you would encounter while doing Milky Way photography. The Live View histogram is disabled and you will need to look at an image file to see a more accurate histogram. This is normal, but the point at which the exposure simulation will fail will be entirely camera dependent. Regardless, I am not aware of any camera that can perform an exposure simulation with an accurate live histogram with the exposure settings typical for stars-as-points-of-light Milky Way photography.