I have been a long time Thief gamer since around 2002, playing Thief 2 the Metal Age exclusively. I was reluctant for many years to give Thief 3 Deadly Shadows a try, but I finally did. And I can confirm that this game is nowhere close, not even remotely close to being anywhere near as immersive and polished as Thief 2. Keep in mind that my experience with this game is AFTER installing the Sneaky Tweak mod that supposedly fixed numerous issues with this game not running well on modern pcs using Windows 10.
I can most definitely say that Thief 3 Deadly Shadows is pure JANK. The graphics are outdated DX9 with faded low-res textures with a blueish tone. Player and camera movement are not smooth at all and feel light and jerky. The fov is low and the screen often jitters when moving. The frame rate drops when quicksaving and when there is too much action on the screen. The screen also flashes to the desktop briefly when loading a quicksave. There are also strange shadow glitches, especially when pulling out the blackjack and bow. There are also major bugs such as the sound of your footsteps disappearing, not being able to pick up bodies, and crashing to the desktop during the awful lockpicking sections. Maps are also dull and uninspired and all look and feel the same. You also can't swim or even use rope arrows. Blackjacking is also extremely clumsy and awkward. There also only seems to be maybe two voice actors for all the guards because they mostly all sound the same.
I could go on and on about how truly awful this game is but GOG has a limit to how long reviews can be. It really has not aged well at all and there is simply no point in playing this game when Thief 2 the Metal Age is FAR FAR superior in every way. I would even suggest Thief from 2014 developed by Square Enix over Deadly Shadows. Both Thief 2 and Thief (2014) are highly immersive and well polished gaming experiences in my opinion and Thief 3 Deadly Shadows should be avoided.