Arena was Frontier's standalone, player-versus-player version of Elite Dangerous. It's also part of the main game, but back in February 2016, the developer had a crack at releasing the competitive component as its own thing. Now, a year later, Frontier has yanked the title from both the Xbox Store and Steam, citing poor metrics and a desire to reduce its maintenance burden.
Community manager Edward Lewis outlines Frontier's reasons on the company's official forums:
We’ve been looking at our metrics which suggest that the vast majority of new and regular Arena players tend to play Arena through the main Elite Dangerous client rather than the standalone Arena one.
We’ve made the decision to remove the standalone version of Elite Dangerous: Arena in order to reduce the number of builds that have to be maintained and managed across Steam, Xbox One and in 2Q'17 PlayStation Network, as well as clarifying the choices for new players of Elite Dangerous on these storefronts.
The game has indeed vanished from Steam, with its link now redirecting to the site's front page.
Lewis mentions that Arena itself isn't going anywhere — it's still accessible via the main game and the standalone version will continue to work. Going by Lewis' post, this should remain the case for the foreseeable future.
Frontier pulls standalone Elite Dangerous: Arena from sale [Elite Dangerous, via Eurogamer]