Rockstar games often teased their upcoming games in the previous ones. Therefore, we can assume that the new Rockstar game (supposedly GTA VI) could be teased in Red Dead Redemption 2 (RDR2).
Sam Houser is the creative force behind all games in the GTA franchise. In 2010, he had appeared in RDR1 to tease GTA V. Although RDR1 was released 11 years ago, the similarities between the revelations seem uncanny. If you are one of the confused folks and in need of some context, you can check out this video.
Rockstar has been quiet about the developments of GTA VI for quite some time. The players are speculating and coming up with multiple theories from the previous games. A YouTuber named Strange Man has come up with a great explanation for the same. In essence, this video covers most of the evidence which could lead to a plausible tease of GTA VI.
You get to read between lines of the letters presented in RDR1 & 2. It only goes on to prove that the speculations around the game might be true. Or, we might be reading too much into it because the fans can hardly contain their excitement, and there might be no such thing at all, according to Dexerto.
Fans have also expected it to be a new development based on the rumor that GTA V might be re-released on PS5 and Xbox. The revamped version is believed to have new and enhanced features. Nothing sure, but we can expect a few more teases of VI in this new version of GTA V. Till then this new discovery could be our closest hint towards GTA VI.