SOMA, the upcoming game to be released, does have Custom Story support. With that said, I don't know how popular it will be. It's not an "Amnesia" title per se. But I can see Let's Players trying it out and advertising the game to get it going. Whether the audience realizes that CS support is there or not may ror may not change the publicity of which your CS will receive, and whether you can create given the assets which are provided.
You can develop your CS and also not be sure whether it will be compatible with SOMA.
I am slowly working on a few maps in the possibility I can release it after SOMA is released if I choose to.
Understand as well that while Amnesia may not be as popular as what it was say, three years ago, people do sometimes still come back and play either Custom Stories or the main game itself. You go to the Tech Support forum here and you'll realize that while people are having problems, it can be treated as an indicator as to how many people still buy or play the game.
If you want to make or continue creating a CS, go for it from my perspective. Even if SOMA is released, there are still people here who will play it, regardless of popularity or SOMA's release.