Update: Riot has announced that One for All is coming back in May with a twist.
League of Legends's "One for All" mode isn't gone forever. Riot Games says that they plan to bring back the experimental game mode in the future.
"It will be back, no announced details yet on when though," said lead champion designer "Meddler" on the League of Legends forums.
In "One for All" mode, all five players on each team play as the same character. Both teams can even play as the same character, resulting in some very confusing matches. Here's a video of a match with ten Karthuses (Karthi?):
Certain champion abilities were tweaked to allow for better cooperation between duplicate champions. For example, the healing blobs dropped by one Zac can be picked up by any friendly Zac. The matchmaking queue was restricted to Summoner's Rift but all maps were available for custom matches.
"One for All" was available in LoL from November 22nd to December 2nd. It was the first Featured Gameplay Mode released by Riot. Other Featured Modes implemented since then include "Showdown," which featured one-lane matches for two or four players, as well as "Hexakill," which allowed for 6v6 matches.
"Cycling new modes will let us work with you guys to create new experiences that would have been difficult to do as permanent game features, and it gives us free reign to go mildly crazy," designer Richard ‘Brackhar’ Hough said last fall. "Also, cycling new modes in and out will allow us to keep these things fresh and focused from a gameplay perspective."
It's unclear whether "One for All" mode will become a permanent part of the game. When Riot first announced featured modes, they simply said that popular modes would be brought back. It's possible that "One for All" will simply stay in the rotation for feature modes, returning for a week or so and then disappearing again.
Riot's other future plans for League of Legends include a new Team Builder matchmaking system. Team Builder allows players to queue up for a specific champion, position or role and find compatible teammates. They recently held a two-day beta for the Team Builder so its official release could come soon.