How can I smooth edges but keep faces untouched? How can I keep 1 face flat but smooth everything else?

I want to keep the line from the first picture. I tried auto-smooth but it doesn't work. It works with shade smooth and edge split, but shade smooth its not enough to give me this effect, I still need to apply a subdivision surface and when I do that, the line also disappears because that specific face is smoothed too...
While the best would be to find a topological solution, Shade Flat and Shade Smooth can also be applied on a per face basis in edit mode.  This can produce some strange results sometimes.