Is a cellular plan required for Apple Watch 9?

Well ... you do not have to have a cellular plan on the watch if you are pairing to an eligible iPhone. You can use Bluetooth but will need to keep the iPhone nearby.

But ... you will not be able to pair that Apple Watch 9n to your iPhone 6. That watch requires an iPhone that can update to iOS 17. Yours is too old to update to that iOS. You would need one of these iPhones in order to use that watch.

iOS 17

• iPhone XR, XS, XS Max
• iPhone 11, 11 PrO, 11 Pro Max
• iPhone SE Second Generation
• iPhone 12, 12 Mini, 12 Pro, Pro Max
• iPhone 13, Mini, Pro, Pro Max
• iPhone SE Third Generation
• iPhone 14, 14 Plus, Plus Pro, Pro Max
• iPhone 15, Plus, Pro , Pro Max

You can also exercise and have the Apple Watch track your activities, without taking your iPhone with you, and when you get home, it will sync the data back to your iPhone as soon as it is in range.

I have had a GPS only Apple Watch for 5 years, and it does everything the cellular model does, as long as my iPhone is nearby.