The FibLib library allows you to "lie" to clients about the state of blocks in the world. This grants developers the ability to send modified block data to the client based on context, which allows for things such as anti-xray, block stages based on user progression, hiding rare blocks, and more!
FibLib was created by Haven King and is currently maintained by Draylar.
Setup & Installation
Visit the FibLib wiki for information on configuring "Fibs". Game Phases also provides advanced examples of utilizing FibLib to lie to players about blocks depending on external conditions and data.
License
Modern versions of FibLib are licensed under MIT.

