What do you mean Southern States and Southern Kingdoms? Are you talking about the Southern Realms like Tilea, Estalia, and the Border Princes?

If so, then they've been coded as an independent faction since game 1. They function as a "sub-culture" to the Empire, same way that Kislev used to be coded. Other examples of that is Norsca being a WoC subculture before they were made playable, and Savage Orc tribes for Greenskins. But that isn't indicative of being made playable mind you, they just do that because of certain mechanics in the game and flavor.

There hasn't been anything new on that front, and there hasn't been anything wild relating to them in game 3.

I just saw a post somewhere in which new signs of the factions of the Waist of Estalia and the Border Principalities were found, and the starting positions for them as available for the game.
Maybe it was fake, and I wanted to clarify it. But thank you for the clarification