iOS 16 beta runs fine with my watch 7 on watchOS 8 which is now technically an old version of watchOS as far as my phone is concerned. I have an unpaired watch 2 knocking about somewhere which I’ve not tried with iOS 16 yet, but based on past experience older watchOS with newer iOS should all be fine. What you can’t do is new watchOS with old iOS, e.g. watchOS 9 will always require iOS 16 or newer in order to pair.