(a.k.a. Original Thief, Thief: Gold, or The One With All The Monster Levels.)

It's been in development for about 7ish years by a team of veteran Thief modders, and finally released a few weeks ago.
Release Trailer
I've been slowly playing through it, and as a staunch fan of Thief 1 (and 2, and 3's good bits), can say it's a tour de force of classic Thief design. The missions are massive, varied, staggeringly detailed, and packed with all the atmosphere and little touches of polish you'd expect from a golden age mainline entry, but bigger and more on account of running on modern hardware.

The original games took a little while to work up to the really killer missions; The Sword, Song of the Caverns, First City Bank and Trust, The Cradle, etc. Here, it's pedal to metal from moment one; a sprawling, winding city district to navigate through en route to an old contact, complete with rich mansions, poor tenements, guardhouses and gang lairs to pick through.

I'm playing on Normal for the sake of replay value, and being pretty meticulous about it, but still routinely leaving stages with thousands in gold still unpilfered. We're talking entire well-realized sub-areas that are both optional and missable; I'm pretty sure I walked past a whole theater during mission 2, and still spent a solid hour exploring before making good my escape.
It even has a full suite of voiced and animated mission briefings styled after the original games. Rather than try to ape Stephen Russell's iconic Garrett performance, it's framed as a prequel with the player cast as Hume: a decidedly less scrupulous ex-con with no ties to the Keepers, fresh off a lengthy prison term, and looking to get back in the game. He's pretty good; rougher edged and less charismatic, but on-theme and convincing, much like the art and sound design.
So, if you like really good stealth games and this wasn't on your radar already, it should be! All you need is a copy of Thief and the TFix Lite patch (or regular TFix, with the default mods dummied out in the CFG), and you're good to go.
If you want to see more, Mandalore put out a highlight video that sells it well without going too heavy on spoilers.