Gameforge’s statement says that the company would have continued on if the developers had not made this decision, but things will move ahead for closure on June 30th. The servers will be shut down that day at 10 AM CEST / 4 AM EDT. Soon afterward, the community, forums, and related social media pages, will also close.
TERA launched in the West in May 2012, and by the time the shutdown happens, will have been running for a decade. In the closure statement from Gamforge, they announced that they will be running permanent events through the final months. There's also a shutdown FAQ page that has some important dates to know if you are still interested in playing TERA through its final days. After May 31st there will be no more new account registration or Thaler purchases. TERA Club purchases will also be deactivated that day.
If you do have any remaining Thalers in your account, there will be no refunds, so you can get in there until the shutdown and spend them. TERA Club memberships will be able to qualify for partial refunds post-shutdown, however.
There are no details just yet on what the events to come for the final months will look like, but expect more info from Gameforge soon. For those who have found community in a game, especially one that has kept going for a decade or longer, a shutdown is the end of a chapter. While some have later found new homes in different games (or sometimes in private servers), that there is warning and time to get in a final couple of months is better than a sudden disappearance.