Don't even need that since the Raboot farewell fake out episode kind of showed he wants to be with Goh.I don't get how Cinderace being Goh's means without a doubt he won't be staying further when with how Cinderace story has been setup they could easily have Goh release him to take care of some mons or something and begin the next series anew with a gen 9 Mon
I think the major concrete evidence is that they seem to have set up an endgame for him with Sonia and that project mew is one week
But using the Cinderace reason is lame as hell and a weak reasoning to be so close minded that everyone who's saying he might stay are wrong
We are talking about the bunny who went a long way just to reach Goh and become his pokémon and then stick with him even after the initial refusal.
The current setup leads me to believe that if, for some reason, Goh needs to be send off to gen9 region with Ash and not be allowed to use his previous critters, they'll just have Cinderace become the Bulbasaur of Cerize lab. We get a believable way of having him not be around Goh and double down on the whole gimmick of Ash and Goh mirroring each other.