I am updating the icons in my app. After I heard Apple released an icon font named SF Symbols
with iOS 13, I was wondering if I can only use them in iOS 13 or if it is possible to use them in lower versions of iOS too.
If I want to use them, do I have to implement a fallback for older versions?
You can not use SFSymbols natively in iOS versions older than iOS 13 (see Apple's Human Interface Guidelines for SF Symbols).
However, if you are interested in using the graphics you can use the SFSymbols app to export SVG versions of the icon. Then use some graphics tool to convert them to PNG icons you can import into your asset catalog.