The exploit which was fixed in iOS 16.2 provided a way for a developer to change system font on iPhone.
Zhuowei Zhang shared a story behind his proof-of-concept app. App itself is available as source code on GitHub.
Apple Support: About the security content of iOS 16.2 and iPadOS 16.2
GitHub: WDBFontOverwrite
Reference: Developer uses iOS 16 exploit to change system font without jailbreak