For macOS, Video Screenshots is currently available for devices with M1 (and newer) processors which generally includes Mac devices released since 2020 (you can see a specific list from Apple here, which also includes how to identify which processor your Mac has).
For iOS, Video Screenshots is currently available for devices with Apple’s A12 processor (and newer, which also includes their newer M1+ devices). A12 devices launched in 2018, starting with devices like iPhone XS, XS Max, XR, iPad Mini (5th Gen), iPad Air (3rd Gen), iPad (8th gen) and all subsequent newer generations of these devices. You can check which iOS device model you have in the iOS Settings app under General → Model Name, and if you want to look up your device’s specs, you can check out Apple’s Tech Specs page.
We’re testing and evaluating performance of Video Screenshots on earlier versions of Mac and iOS devices, but in general, the devices that don’t have this feature haven’t been able to reach sufficient performance levels to properly support the feature. We’re continuing to evaluate this and will add older generations of devices when possible.