Anybody know if watchOS 8 is supported by iOS 16?

My girlfriend has my old Apple Watch Series 3, so presumably it’s on watchOS 8. But her iPhone is eligible for an update to iOS 16.

If we roll her phone forward, will the mismatch cause a problem?

Yes, it is. I’ve had IOS16 betas on my phone since early July and a Watch S4 on watchOS 8 until this week. No problems