FusionAesthetic
Sci Fi Trainer
At first, the writers seemed to follow a pattern of giving the previous Pokegirl a cameo in the newest series but lately they seem to just do whatever they want regarding past cameos. I was surprised to see Misty and Brock in SM, honestly I was surprised to see Gary in DP.
I think there's no definite answer because for all we know, the writers could randomly give Iris an appearance in Gen 8 just because. Honestly who knows..
Pretty much this the writers have dropped patterns (which might be good?) meaning anything can happen.