building=skyscaper is not a tag in common use (<20 uses) and not documented on the wiki, nor have I seen any discussion on the tag.

In general I don't think this is a good idea, as "skyscraper" is hard to define, you'd need to pick an arbitrary height and then you may as well just set the height=* tag directly. Furthermore it removes the ability to define what kind of building it is, eg. residential tower or office building.