It depends on what city your are in. As you know there are 93 cities in St. Louis county.
One city may allow 3 chickens, someone across the street in another city may be allowed 15 chickens, there may be another city at the end of the street that allows unlimited chickens AND roosters.
If you are in unincorporated St. L. county, yes, they are legal, but you aren't allowed to breed chickens. Weird, I know.
There are several cities that allow chickens.
I teach chicken classes at St. Louis Community College. I'm supposed to have a beginner class coming up April 4. That will likely be cancelled now.
If you message me I can deliver much more information if you give me your phone number.