The regional unit Δημοτική Ενότητα Κορεστίων, created in 2011, is not on OpenStreetMap. This is the most important name now for the region: when one turns off the primary highway into the village of Νέος Οικισμός, the highway signs say “Κορέστεια”, not “Νέος Οικισμός”.
Administrative divisions in a foreign country are too difficult for me to get correct, but could the local Greek OSM community add this?
Many municipal units are still missing, as well as communes. It’s not easy to add them all in a short time, it takes time to draw the borders and it’s not a high priority. From administrative perspective, municipalities are the most important to have, and then comes their subdivisions (municipals units wherever exist, and communes).
I have seen routings apps use such names for places, and I’m pretty sure they don’t take such data from OSM, because it wouldn’t be named as “Κορέστεια” but as “Δήμος Κορεστίων”, because those apps actually use the pre-2011 municipalities names. They show the name of the administrative area and not the settlement, because they follow the practice from other countries where they refer to a region based on the administrative name and not the settlement one. That’s why in some routing apps, the area’s name doesn’t make sense sometimes.
Edit: I just realized you meant the highway signs, not the signs on routing app, sr for the confusion 
I see that Κορέστεια is a historic region, and the highway may denote specifically that and not the former municipality. The municipal unit may cover the area of that historical region, but it isn’t the same thing. Yes, the municipal unit, when it will be added, will denote the historical area at the same time, for those who know about it, I’m not saying that additional tags will be added about the historical area.
I was there a couple of weeks ago, and I no longer remember if the highway signs said “Κορέστεια” or “Δήμος Κορεστίων”. It could have been the latter. What is important is that the highway signs did not say “Νέος Οικισμός”. My initial thought was that the name of this one village had been changed. Fortunately I understand Greek well enough to read the Wikipedia article, but I think other foreign mappers surveying in this area will be confused unless the name of the larger administrative unit is also added to the map.
I will add it the next days, but regardless, it’s from the cases where the place name on the highway signs doesn’t refer to a specific settlement, but in the region as whole. Usually it’s the villages that are on the signs, but if the region is well-known, as part of the regional history, then it may be mentioned on those signs.