It’s not even antiquated depending on where you live.
Mumbai is a corporation.
So is San Francisco: a municipal corporation with a Charter. It’s an overlooked detail, but it’s not just a choice of phrasing. Corporations are just groups of individuals acting as a single entity under law, and incorporated cities in California and most of the States I’ve looked at are no different in that regard.
It is not antiquated, but merely a descriptive that has fallen from common usage. ~ Every local authority in the UK is a corporation and up until c 1970 is was common to see 'Wherever' Corporation buses in British Cities.