Hey,
So a lot of offers are still promoting iPhone 6, 7, etc. However, I don't see these older model offers on Adplexity. I mostly see iPhone X, Xs.
I was thinking about running iPhone 6,7 offers and just changing the X to a 6 or 7 and not doing anything with the graphic of the phone, i.e. promoting an iPhone 6 offer with iPhone X graphic.
So my questions is, is it worthwhile to spend money on the older model offers while using a iPhone X graphic? Or maybe I'd need to change the graphic to the respective model,e.g if its a iphone 6 offer, find a 6 graphic and use it.

As Matej said they might be outdated landers from lazy affiliates or cloaked ones.
Your funnel should have some continuity.
Same as with adult dating.
If there is a black skin girl on banner, but then on lander is a redhead and on the offer is a blonde girl there is no continuity in the flow and you don't really know what to expect.
Of course all these inconsistencies will still get some amount of conversions, but making the transitions through your funnel more consistent can bring you more gains.
It makes more sense and it's more believable to your users.
I'd also add that if you are not finding iphone 6 and 7 offers in Adplexity - it might be because they are not making money.
Some affiliate networks will have had those iphone 6 offers available for years. It does not mean they are still making money.
They might .... but probably not.
If unsure, just ask you affiliate manager if anyone is doing volume on that iphone 6 or 7 offer before spending time and money on it.
I'd suggest that you might want to take a bit longer and look for an Affiliate Network doing iphoneX offers and look to promote those.
Even better if you can track down the iphoneX offer showing up in Adplexity.
Yeah stick to the new model offers they're the best, test a few of them though as sweeps vastly range in conversions. Try Gotzha, yepads or Wewe