Present requirements for iOS 13 include Mac OS 10.11.6 for iTunes 12.8 as a bare minimum.
On September 19, 2019, iOS 13 was released. Including a method to downgrade to iOS 10.3.3, and a method of downgrading to iOS 11.4.
This tip is designed to be a one stop shop to find out what iOS you can use if you are able to install a specific Mac OS X.