In late 2016, Warner Bros. Games announced "the online services portion of Batman: Arkham Origins will be retired". And that was it for Arkham Origins' multiplayer. It's tough to say it was hugely missed, but it did have an intriguing concept. Two teams of Joker and Bane goons fought each other, with the chance to play as their gang's leader if they performed well, Star Wars: Battlefront-style. Meanwhile another player got to be a predator in the shadows as Batman (with or without Robin).
That said, many of the players who complained about its loss on the Steam forums seem to be achievement hunters frustrated they've been unable to snag all the multiplayer achievements since 2016. Either way, modders were motivated enough to bring it back. You can download a mod that restores the multiplayer, and adds some singleplayer skins to challenge mode and free roam, from Nexusmods.
If you don't care about the skins, another way to restore the multiplayer is to go to your Steam\steamapps\common\Batman Arkham Origins\Online\BmGame\Config directory and edit the DefaultWBIDVars.ini file. Find where it says BaseUrl="https://ozzypc-wbid.live.ws.fireteam.net/" and delete the "s" out of the URL so it starts "http://". That's it, that's all you have to do.
This will mean you're not connecting to a secure site—that's what the "s" stands for—so exercise caution. If you're looking for other players to coordinate with, the Batman Arkham Discord server is where they're currently gathering.