"True Detective" having a new sequel would now depend on Nic Pizzolatto's interest in continuing it, according to BreatheCast. There are also other rumors that popped up last April, when HBO enlisted "True Detective" as part of its "Complete Series" list. Due to this, some fans believed that the advertisement for season 3 is false, while others state that it's only "complete, season-wise".

Pizzolatto, the creator of the television crime drama series, just recently signed an HBO contract expected to last until 2018, reports Christian Times. This contract might just mean that season 3 of the series will happen, but it might take until 2018 for it to be released. HBO has given Pizzolatto free rein in what he wants to do with the series, whether he will work with other writers or showrunners, or run the entire show alone. However, the award-winning writer hasn't responded yet, even though apparently HBO is expecting him to deliver a response on that soon. Whatever he chooses, he will remain as the executive producer for the series.
Vine Report also commented on this, since it seems like season three hasn't even started on the pre-production process just yet. There has been an interview with Michael Lombardo, the boss of HBO, where he explained the problems they faced during season 2. He said, "I'll tell you something. Our biggest failures — and I don't know if I would consider 'True Detective 2' — but when we tell somebody to hit an air date as opposed to allowing the writing to find its own natural resting place, when it's ready, when it's baked — we've failed. And I think in this particular case, the first season of "True Detective" was something that Nic Pizzolatto had been thinking about, gestating, for a long period of time. He's a soulful writer."
He further added to this, by saying, "I set him up. To deliver, in a very short time frame, something that became very challenging to deliver. That's not what that show is. He had to reinvent the wheel, so to speak. Find his muse. And so I think that's what I learned from it. Don't do that anymore."
While the second season was not that stellar, several still believe that another season of 'True Detective' would still provide something interesting for the crime genre. Ecumenical News mentions that this silence could just mean that people involved are keeping it under wraps, so it is best to stay positive and wait for HBO to announce more on their progress.