The suits in question are the Anime Batman suit and the Zur En Arrh suit, and starting today, after you update Arkham Knight, the suits will be unlocked and playable for free, without any hoops to jump through.
With both the forums and WBplay gone, players have been asking Rocksteady and WB Games to patch Arkham Knight and restore access to the skins.
Today their prayers were finally answered, as with little warning or teasing, Rocksteady announced that Arkham Knight will be updated today to make both suits freely available to all players.