NetHack is a single player dungeon exploration game that runs on a wide variety of computer systems, with a variety of graphical and text interfaces all using the same game engine. Unlike many other Dungeons & Dragons-inspired games, the emphasis in NetHack is on discovering the detail of the dungeon and not simply killing everything in sight - in fact, killing everything in sight is a good way to die quickly. Each game presents a different landscape - the random number generator provides an essentially unlimited number of variations of the dungeon and its denizens to be discovered by the player in one of a number of characters: you can pick your race, your role, and your gender.
It's not. It got a new version, v3.6.1, this year. The source tree is very active and even has a GitHub mirror. They made an update to the code literally 5 days ago that fixes a bug regarding "AC tracking with negative numbers". One of 15+ patches that are slated for release in v3.6.2. And if you wanted to, say, play the bleeding-edge v3.6.2 right now, you can, by doing a git clone and compiling the source code.
Also, if you look at the site news history, you'll find that it's been updated regularly, with the longest periods of silence being about three years long. The decade-long dead period from 2003-2013, that these people speak of, does not exist. Plus, three years of silence on the news end doesn't necessarily mean the dev wasn't working on the game. It's possible that the patch that broke the silence was a huge update that legitimately took three years of listening to bug reports, playtesting, R&D, etc. until he felt that there was enough substance, and the code was stable enough, to finally push a new release. If that's the case, there was probably a way to play the "bleeding edge" or "public beta" on the website. And it was probably that second git mirror.
In fact, that's pretty much exactly the case. If you read the old news posts, the dev makes it pretty clear that he's always working on it--for instance, take the news post from 2014 about the fake release incident that prompted the dev to skip 4.x and 5.x and go straight to 6.x to prevent confusion. There's a difference between very long release cycles (which is what NetHack has) and project abandonment.
Please update your listing.