Soraka is, both pre-retcon and after. She is the only one, lore-wise, to have been said to reach such a level. The implied meaning was a god. She had to give up her god status to walk with the mortals in the retcon. The old Soraka ascended to said status. The only potential equivalents would be those of The Void, where it appears to be an extra dimension/universe (the way I look at it). I would venture to say there are those in The Void who are more powerful than anyone else in the universe where Runeterra exists.. combined.
And @ celestial entity/being =/= deity. Technically, you're right - it doesn't always. But you cannot use DnD to support that because it's lore and what celestial means is completely different. Generally, if something is said to be celestial, it is accepted as a divine or a deity. Even in the case of it not being THE divine/deity, it is usually immediately as close as possible (in the case of an angel/etc). Celestial being can also have a lesser known sense where it can refer to space and the universe/stars. I, for Soraka's lore, see it as a combination of a deity and stars.