Do the Shadar-kai even need an article based on them? Yes they appeared in 4e, but I never had the sense that they had gained any sort of traction whatsoever.
If anything, out of the creatures created by the 3e Fiend Folio, I would have greatly preferred the ethergaunts or the kaorti. Those two were awesome, the shadar-kai... didn't strike me as anything special, and I don't really care to have something as overtly evil as the shadar-kai attached to Shadow. But 4e's design team liked the shadar-kai, so they had lots done with them (I think Mona and Jacobs created the ethergaunts and kaorti respectively, so they weren't there to push for their creations with 4e, which is unfortunate because it meant little was subsequently done with the most awesome monsters in that book).
I just don't really see shadar-kai as anything remotely resembling iconic for the Shadow Plane. I'm largely ambivalent about them, but if 5e heavily pushes their 4e origins (the meta-origin of the Raven Queen ruins it for me to an extent) it isn't going to do the edition any favors IMO.
Shadow deserves denizens, but the shadar-kai aren't anywhere near the top of the list in my opinion.