Prior to 3.1.x the dat subdirectory didn't exist yet but data.base was present somewhere, possibly in the src subdirectory instead. I don't know whether it goes as far back as hack 1.x or even nethack 2.x. The source code for a bunch of old versions is present at https://nethackwiki.com/ but I'm not sure whether the data files are all there too. The source code for the most recent released version (3.6.7) is... Source: 12 months ago

I am not sure what extra benefits the Steam version has, but the game is available for free either as download on nethack.org, or to play online at something like alt.org/nethack or hardfought.org. Source: about 1 year ago

You don't even need two people to play D&D, Nethack is great! Source: about 1 year ago

I've always been fond of the last quote on nethack.org. Source: about 1 year ago

It was discussed here last fall, when the home page for nethack.org was updated to mention it. The blurb includes a few other tiny screen shots and says the exhibit is scheduled to be open until mid-July. Source: about 1 year ago

Terraria - This classic title has been around for a very long time. It's a game about exploration, base building, and adventuring in a sand box world filled with bosses. I haven't played much of Terraria, but if I remembered correctly, there are various classes you can choose to build your character into, such as a summoner, wizard, melee/dps, etc.. Another game with a similar element of play, but focusing more... Source: over 1 year ago

There's also something like Terraria and Starbound if you enjoy going on adventures with a bit of a RPG element as well as building. Source: over 1 year ago

Starbound has one of the best soundtracks of any video game I've played. Source: over 1 year ago

Starbound: Includes a race called The Glitch, a medieval society of robots created by persons unknown in an experiment to understand how societies form. I'm not actually sure if the post-beta builds did anything with that lore, but nevertheless you can play as one and find settlements of them out in space. Source: over 1 year ago

Terraria and Starbound (also great titles to play) is a pixel RPG adventure with build elements. Most of it is focused on the adventure/RPG with the building being a minor element to the game. Source: almost 2 years ago