After publication, Samsung told us that the reason for the S21 Ultra's digitally upscaled slow-mo video has to do with the phone's larger image sensor, which operates at a lower shutter speed when recording video. Rather than artificially limit the smaller phones, which can reach a higher shutter speed, Samsung elected to give them the full 960fps recording capability and use its frame rate conversion tech on the bigger phone, implying that the difference between the two wasn't very perceptible.
I know this sounds like an odd explanation, but so far as we can tell, it's legit. This DP Review article detailing the difference between shutter types claims that the performance of electronic shutters (as you'd have with smartphone video), can be affected by not just pixel count, but pixel/sensor size, and the S21 Ultra does have a bigger primary sensor with a very high pixel count compared to the smaller S21 and S21+.
