A large portion of the content in Stray is found in The Slums. Two of the game’s chapters are set in this area. That said, once you leave it for the second time, there’s still a way to go back.
How to Go Back to The Slums in Stray
The doors to The Slums will be closed behind you after you leave for The Sewers. However, there indeed is a way to go back to the area. That door will remain closed, though, because you can’t actually go back to The Slums through in-progression means.
Instead, you’ll have to go back to the main menu. First, select Start Game. Then, choose your save slot and you’ll find a Select Chapter option below.
You can do this at any point in the game, including after leaving The Slums completely. This is a perfect opportunity should you wish to return to the area to collect and discover any items, collectibles, and memories you may have missed.