Watch Dogs Legion's multiplayer was delayed on PC indefinitely a couple of weeks ago due to issues with "certain GPUs". Those issues have now been fixed and so online mode has launched in update 3.22.
You can find the full patch notes on the Watch Dogs Legion forums. The update is a 14.4GB download, and the tweaks alongside the online mode are mostly just fixes.
Legion's online mode is focused on co-op play, letting you team up with pals in a new instance of London to recruit Dedsec members and carry out dedicated co-op missions with up to three others. Ubisoft are also going to be adding new Tactical Ops missions on March 23rd, which are five-stage challenges that require more co-op coordination to complete.
Finally, there's the Spiderbot Arena, which is arena deathmatch where you control one of the game's little hackable spiderbots. It looks throwaway, in the way additional deathmatch modes always do, now.
The original Watch underscore Dogs wasn't a great game, but its multiplayer was the best thing about it. It let you hack into other player's singleplayer games, and a lot of its modes involved hiding in plain sight among NPCs while another player hunted for you - or was completely unaware you were there. I'm a little sad those modes aren't returning, but I do like co-op, too.
If you're still muddling your way through the singleplayer game, check out our Watch Dogs Legion guides for a little help. And if you don't own the game at all, it's currently on sale with 50% off in Ubisoft's spring sale.