Great news: the source code for the original System Shock has been released under the GPLv3. It's the source code of the Mac version, but that's irrelevant since it's the same game as the DOS version, and it's for classic Mac OS, so it wouldn't run on modern Macs anyway.
https://github.com/NightDiveStudios/shockmac

So what does this mean? For now not much, but it can be awesome in the future. I don't know what Night Dive has planned to do, but anyone can now write a source port. You will still need the game assets (graphics, sounds, maps) for the original game, so it's not freeware, but it is Free Software in the sense that users have the freedom to study and make changes to the software. People could add modern features like higher resolutions, more intuitive controls, proper mouse look and even more to the game.

There is already the System Shock Portable. Maybe that could be further enhanced now but I guess that is just what the SS Enhanced Ed sold here is.

This mac-source is a release mainly for "archival" reasons, as per messages on Discord.
ND is also planning a source port release, which has already done the legwork of porting the old base. This release will also be the basis for a new updated version, that owners of the current "Enhanced Edition" will get for free.
Recent sources:
* https://www.reddit.com/r/systemshock/comments/8a47kf/system_shock_source_code_released/dwvx4dt/
* https://www.reddit.com/r/systemshock/comments/8a47kf/system_shock_source_code_released/dww7ell/

Furthermore, for anyone dreaming about working on the (future) port, on the systemshock.org forums we've created a dedicated sub-section for this. Mainly to avoid many different port-attempts at once, and focus all on one place. See here:
https://www.systemshock.org/index.php?board=13.0

That's nice, too many games had their source code lost and the best people can do is patch old binaries with duct tape and bubble gum. That's even more nice. The Enhanced Edition is OK for something hacked together by one guy through reverse-engineering, but it's nowhere as smooth as a modern source port for a game like Doom. I hope they release the source of that port as well. I don't know why they didn't just do this in the first place, I honestly don't care about the remake. The original graphics are fine, the interface and controls are what is clunky. That's really cool :)

Just got an email from Nightdive, the source port is in beta! Hope GOG gets the update soon when its final.

Stephen Kick mentionend that he plans to release the System Shock 2 source code:

https://twitter.com/pripyatbeast/status/1042674927793401856

yep, linking the systemshock.org discussion - the gist of it is that this is good news for people who wanted a native linux/mac version, and bad news for people who wanted mods on steam.

I think icemann is right. NewDark or New NewDark or whatever it has been called in the past is a dead end for the future of the game. A closed source unofficial patch with irregular improvements by someone who wants to stay anonymous is definitely worse than an open source solution that has many contributors.

The only bad part that I can see is that NewDark's developer apparently does not want NightDive to release his/her changes under the GPL. But I'm sure that a future open source release will catch up soon.

Take a look at Shockolate's improvements during the last two weeks: Working XMI music playback and rebindable keys are the most striking changes, but there many more small improvements.