Perhaps the biggest breaking change comes from a single word change...
Fabric API 0.79.1 was released, with breaking changes to Object Builder (FabricBlockSettings). This was followed by another patch update, 0.79.2, which fixes a mixin error.
Also, Loom 1.2 is out, with some changes to library handling, Kotlin support, and minimum Gradle version bumped to 8.1.
Entity#world is now private. You can either use an access widener (the cheaty way) or replace it with getWorld() (the good way). Since it is private, subclasses must also use getWorld. If you are in a server context, you can use getServerWorld to get the server world. If you somehow need to set it, setWorld method also exists.
Entity#onGround is also private now; use the getter instead.
Material class, stripped of most functionalities in previous snapshots, is gone. Map colors should be specified using block settings.