In reply to an inquiry on the Tribes Reddit, Hi-Rez Studios confirms that they are not planning any development on Tribes Ascend now or in the near future.
Fan and Reddit user HiRezTodd posed the inquiry in a very particular way. He pointed out that the last patch on the game was released four months ago and they have not received word since then from contact persons APC and Kate. He basically requested that the company give the official word that they've stopped development on the game.
In their reply, Hi-Rez Studios confirms that they have stopped working on the game, barring Kate working on a few additional maps. The company is currently busy on SMITE, and after that will drect their focus on Global Agenda. If ever, Hi-Rez would work on a new installment of Tribes Ascend instead of adding to the current game. However, they also have no current plans to make such a sequel.
Upon researching this piece, I verified that there were some more possibilities Hi-Rez did not address in this answer. They once indicated interest in bringing the game over to Playstation 3 and Xbox 360. They were also working on a different Tribes game, and MMO shooter called Tribes Universe, before deciding to focus on Tribes Ascend.
In modern standards, Hi-Rez is a small independent studio, based in Alpharetta Georgia. They genuinely may not have the manpower or capacity to maintain support for Ascend and keep working on new games. This still makes the situation a poor one for Tribes Ascend players, especially when you consider the game is barely over a year old. At one point, they managed to pick up as much as 800,000 players.
Source: Hi-Rez forum