Rumors and leaks about the remaining lowrider vehicles which were added to GTA V's game files with the Lowriders: Custom Classics DLC, but weren't made available in-game, have been circulating for some time now. Even though players knew they were coming, no one knew when they will arrive. In the meantime, the leakers who found this info moved on to release images of game-models regarding sports-related Adversary Modes.

The "hidden" lowriders even made their way to single player after the content from Custom Classics was ported over via mod. With the cat out of the bag regarding the three rides and new Adversary Modes — which are blessed with names such as "Inch by Inch" and "In and Out" — all that remained to be revealed was when this new content will be finally released.
While there were no dates among the leaks, it seems that the new content will be coming to GTA Online one by one, on a weekly basis. At the moment it looks like we'll be getting one lowrider per week in April, with a mini-DLC containing the sports Adversary Modes and possibly some new character customization items at the beginning of May.