You can still play it :) as it was in the old days - https://old.marblevortex.com/. Source: almost 2 years ago

Full copy with no expiration, locked levels or restrictions to online play. The team has updates for it still. It runs really well and doesn't require a powerful PC. Source: over 2 years ago

Pc port: https://old.marblevortex.com. Source: about 3 years ago

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: about 1 year 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: over 1 year ago