With the Age of Discovery expansion, you can no longer swing over to the slums of Freeport (e.g. Longshadow Alley) and enter the sewers from them. Almost all access to the slums is gone, with them only open during specific quests such as racial quests.
If you're in the Commonlands, you can head to The Thieve's Way near through a sewer grate located at -1,251.30, -83.82, 119.95 just slightly south of the City of Freeport-West gates. If you're like me, then you can bypass having to go to the Sprawl to go to the Serpent's Sewer in order to reach The Thieve's Way.
Once you're inside the sewer, the grates that lead to the City of Freeport will be in the same general area; North, South, East, West.