You can't, it was removed from all digital services. The only way you can still attain a key for the Steam version is through the grey market like G2A or even Ebay. Purchase at your own risk.
And I gave in and purchased the Steam Edition of 20th Anniversary World Tour. Why not? I already own every other version of Duke Nukem 3D, anyway.
So far... it is solid enough. The port isn't that different from EDuke32 and has less features/ options to tinker around with. Really funny seeing an option for Ambient occlusion in such an old game. The majority of the textures also have low resolution normal maps applied to them, which add a nice touch too. The game plays smooth enough with the polygon renderer, and you can switch between the classic renderer on the fly at any time.
Also gamepad support is pretty well implemented here too. It is still not perfect, but playable. The aiming still feels a bit clunky, but what can you really do? This is an engine port of a 20 year old game that was originally designed primarily for a keyboard and mouse. And that is still the best way to play it. But the gamepad support is still solid enough.
Also this version of Duke Nukem 3D brings in the rewind feature from the Xbox 360 port. This is pretty nice as well. Something that Megaton Edition was going to add to their port, but it never made it in.
New voice acting from Jon St. John is alright, though lacks some of the punch of the original Duke 3D renditions. Though I noticed a few weird omissions like "Damn, that is one Doomed Space Marine".
The new episode is really nice. Great to see some of the original level designers come back for this. Some of these new levels take full advantage of the Build engine and there is some nice new texture work in them too. The new enemy, the firefly could be a little better, but I am not complaining. Lee Jackson's new music tracks are also welcome as well.
I love that a game like this can be revisited 20 years later with new content that was created by the original level designers.
It is a shame though that it is missing all of the expansions from the Megaton Edition (Life's a Back, Duke it Out in DC, and Nuclear Winter in particular). But this is still a pretty good attempt from Gear Box.
But I agree with Clint from LGR that it does not feel like the definitive version.
thanks, im still wondering between 20th Anniversary World Tour and the megaton edition for some coop with friends who never owned or played it. i don't own either myself.