Link I like how you ask the "difficult" questions
Link Say you're watching a movie on DVD/Bluray can temporal dithering be an issue?
Depends on the graphics chip of the device outputting the signal, since "temporal dithering" isn't an item listed on the specifications of devices.
Some people here say the PS3 is good, so I guess you could just use that as a DVD/Blu-ray player.
Link Or say a downloaded movie/tv show on USB through some sort of media device or plugged directly into TV? What about say viewing Netflix/YouTube etc on something like a roku 3 or nvidia shield tv?
It depends on the device itself, some are smart enough (Older nVidia) to not do dithering when plugged into an 8-bit display. Intel and AMD chips are the outliers.
Link Also does the content matter say movie vs cartoon vs animation?
Some content, such as dark backgrounds can bring out dithering more, but other then that, not really.
Link What about viewing on cable box?
Depends on the graphics chip and software of the device.
Link All of this keeping in mind using a true 8bit display.
Again, if the chip has "forced dithering" like AMD, that's not relevant unfortunately.
Also when it comes to phones/tablets are there any that are confirmed not to use temporal dithering/pwm?