Is crossplay confirmed?
No cross-platform multiplayer feature has been announced.
Because GTA VI Online itself has not been detailed, claims about crossplay are premature.
The evidence
No announcement found in the sources checked on the verification date.
Rockstar GamesGrand Theft Auto VI official siteRockstar Games SupportGrand Theft Auto VI platforms, editions, and versionsRead the full supporting guideWhat would change this answer?
A Rockstar online-mode or platform-feature announcement.