You can still grab Crucible on Steam until July 1 at 9 am Pacific, but after that, it'll un-release and become closed to new players. Eventually, there will be a beta signup process available on the official site.
During the return to closed beta, Relentless Studios (which is part of Amazon Game Studios) says it'll stick to its existing development roadmap, which you can see on Trello, while considering feedback from 'beta players'—who were just regular players up until this week.
"For the most part, your experience as a Crucible player will stay pretty much the same while we're in beta," wrote franchise lead Colin Johanson. "One of the biggest changes you’ll see is that we're going to schedule dedicated time each week when we as devs will be playing with the community and soliciting feedback. The game will be accessible 24/7 so you can continue to self-organize matches with other players—we recommend joining our Discord server to find players to queue against."
The reason it'll take organization to get into Crucible matches is the same reason it's going back into closed beta: Not many people are playing it. At the time of writing, Steam Charts shows 146 concurrent Crucible players. That is a disastrous player count for a new free-to-play multiplayer game.
The only recent game to faceplant as hard as Crucible is Artifact, which Valve has been retooling and went back into beta earlier this year. It remains unusual for a game to release without qualification and then slink back into a testing phase, though. Aside from Crucible and Artifact, I can only think of a couple other big examples. The Arkham Knight disaster comes to mind, but that game wasn't free-to-play.
During Crucible's return to beta, Johanson says that Relentless Studios will work with a "community council," which will include casual players as well as those who are "highly competitive." I'm not sure that there are highly competitive Crucible players at the moment, but if you like the game and want to see it improve, joining the council may be an opportunity to influence it.
While un-releasing a game is unconventional, Relentless had to do something major if it was going to have any chance of turning Crucible around—a few patches can't scrape a total flop off the floor of Steam. And where an independent studio might've run out of funding and had to close up shop, Relentless has the financial backing of a company that could buy a few countries if it wanted to. We'll see if the studio lives up to its name.