That's not really true, though.
There's an entire extra campaign you can play in the Special Edition where you go with the Ink Dude (the old man that paints the tatoos on your body)
He has a nice arsenal of non-lethal weaponry but some very lethal too and his mission can also be beat with excessive slaughter.
The base game already features the ability to do non-lethal quite well and you can unlock a suit that removes lethal options all together (but makes you complete silent when running) forcing you to sneak and not kill anyone (except with the environment).
And the best part is, you unlock that suit by performing non-lethal secondary goals.
It's true that you do get the old man's weapons for yourself in the campaign, but they were just 2 or 3 and while they were neat, they were also not that much better than your default kit. The only truly unique thing it adds is non-lethal finishers, but you can do non-lethal without them.
Played it, was really great at some things, not so much in others.
One thing I really loved is that it did the "small thing in a big world" very well. Guards look fucking huge compared to you and when you go to the outside areas, you look at all the space you can walk around and it's gorgeous, especially since you move vertically a lot as well. Graphically, the game is amazing specifically for this.
It also had the cloning mechanic that was pretty neat and let you do some nice stuff. It kinda ruins the idea of 100% pure stealth, though but the ending plot twist using a gameplay mechanic you grow so familiar was genious.
What I didn't liked was something that many stealth games suffer from. Levels end up feeling a lot like some sort of puzzle for you to figure out with solutions conveniently placed around by the Devs that you can spot and use.
I get that this is a philosophy in game design that a lot of people enjoy, but I really don't like reaching a room and seeing the main door with a guard (combat route), a second route by a hole in the wall up some ledges (the sneak route) and a fruit bowl where the guard is always eating (the assassin route).
The fun thing in stealth is studying the situation and coming up with your own plan to beat it, not have these readily available and very convenient solutions for you that are so obvious they might as well make it a QTE.
You'd often see many different paths that you'd try to take only to find they are dead ends because, to pass into the next area or section of this area, you'll have to go through this specif part of the map that is only connected in these very specific routes. It's a bit disapointing.
I'd much prefer a stealth game to give me impregnable fortresses with guards on realistic patrols and architecture that makes sense so I'd have to adapt and use common sense.