Its been so many years we have been playing GTA Online without cross play. But now since GTA 5 and its online version are coming to PS5 and Xbox Series X maybe Rockstar should start thinking about adding cross play functionality to GTA as well.
We have games like PUBG and Rocket League with cross platform functionality so why not GTA as well. Its going to take a lot of background work and maybe there are legal issues involved as well in between Microsoft and SONY. But if Rockstar really wanted that to happen I am sure they would find a way to make it happen.
Rockstar has not announced this feature yet, but Grand Theft Auto 5 will be coming to the PS5 and Xbox Series X|S later this year. If there were ever a time to add this feature–even if only for those two versions of the game–it would be during that development process. We know those versions will have some content not present in the current-gen versions of the games. It’s not clear if something like cross-play would be possible should there not be content parity. (Source)