This is the ability for the user to set the preferred font size inside "Display & Brightness" on some phones and "General > Accessibility" on others. I'm told that many apps on the store don't support this yet, but @abarth seemed to think this would be relatively straightforward for Flutter to do.

https://developer.apple.com/library/ios/documentation/UserExperience/Conceptual/TransitionGuide/AppearanceCustomization.html