It looks like someone just used a little flood fill to create the thermal pad rather than creating a new custom footprint. That would have been fine except that the flood fill area has solder mask on it.
In Eagle, if you want to keep the mask off of an area that would other wise have mask, you draw a polygon in layer 29, tStop (or 30 bStop for the bottom) over the are you want to keep mask off of. Not difficult, but not necessarily obvious either. With Sunstone PCB123, you pretty much do the same thing with the SS Top or SS Bottom layers. I don’t know about any other packages, but I would guess it would be a similar approach.
Of course, just making the footprint with the library package editor would take care of it too, but sometimes it’s just more expedient to take a footprint that’s close and mod it up with a polygon or something similar.
Is Oregon like a Polygon?
No, because it hasn’t “gone” anywhere