Seemingly one was planned but it's been more than 2 years with no word of it, so I take it it's not happening?

In the upcoming days (on 6th of April, according to Discord), Nightdive wants to send out an update on their plans. As per various hints during the previous weeks, this update will also include the timeframe for releasing the source codes (plural) of SS1.
First, they want to release the original source code. Then, apparently not at the same time, they want to release a "source port" of SS1 that they have been working on during the last months as well.

Hints about this source port are sketchy. It appears they intend to replace the current version of the "Enhanced Edition" with this source port, and the intention of this source port is to make it available to modern systems.
Whether or not this also includes support for other OS out of the box is unclear. After it has been released, I suspect it wouldn't take long for the community.

While the original source code would people allow to port it as well, this work would most likely be a duplicate to the source port.

Resources:
* https://www.reddit.com/r/systemshock/comments/7x8rd1/ss1_source_code_to_be_released_in_two_versions/
* https://www.systemshock.org/index.php?topic=8642.msg117808#msg117808
* Info collected on my own from their Discord channel

Oh wow, that greatly exceeded my expectations. Thanks a lot for the very comprehensive post.