The mud block is a part of Biomes O' Plenty to make its swamp-like biomes more realistic. It uses the slime block properties (including bounce, but not much. It has the slime block sounds as well.).
It was removed in 1.19 due to the addition of mud to vanilla Minecraft. The mud that would normally generate in Wetlands, Bayous, and Glowing Grottos was replaced with vanilla mud.
Crafting Recipe[]

Mud Block