WHEN?

No, but seriously. It's been almost  a year since trailer dropped.

Was it ever really promised?

There has been a thread about this before and then one of the devs answered this:

The-Cat-o-Nine-Tales wrote:

We love the song in the trailer as much as you do, and we certainly want to share a full version with all of you eventually. However, we are not ready to share it yet, and we also can't tell you yet how that will happen: It might be part of the downloadable soundtrack, it might just be a youtube upload, or any number of other options.

So, stay tuned! :)

So yes, they would definitely like to share the music with us according to this post but keep in mind that it's not so simple with professional artists like Lera Lynn and their publishing companies. Amplitude Studios doesn't necessarily own the rights to the music piece so they'll have to negotiate on how and where this will be published. And of course the timing will be another factor. Dropping the music piece right now might not be the wisest move on the marketing side given that only 4 months ago they dropped a first small episode on how the game will even function.

Therefore release isn't going to be anytime soon and I'd think they'll release the music once the release date is quite close. Or there's a possibility that they might not even release the music ever if there's a problem with them and the publishing company. Althought when going trough Lera Lynn's Spofity page there are relased tracks from few TV-series where her music has been featured on.