Curseforge user DcNdrew points out: https://www.curseforge.com/minecraft/mc-mods/water-erosion?comment=69

Biomes O' Plenty dirt doesn't work.

Water Erosion Forge and Fabric versions will have slightly different Custom Block systems and will be unable to use common code. I wish the BlockTags class had something to identify Dirt like Sand and Leaves do.

Docs:

• Forge Registry: https://mcforge.readthedocs.io/en/latest/concepts/registries/
• Fabric Custom Blocks: https://fabricmc.net/wiki/tutorial:blocks

A Fabric code example:

import net.minecraft.util.registry.Registry;
System.out.println(Registry.BLOCK.get(0));
# [STDOUT]: Block{minecraft:air}

A Forge example from TerraForged:

• https://github.com/TerraForged/TerraForged/blob/c6eec525/TerraForgedMod/src/main/java/com/terraforged/mod/material/MaterialHelper.java
  https://github.com/TerraForged/TerraForged/blob/c6eec525/TerraForgedMod/src/main/java/com/terraforged/mod/material/Materials.java#L51-L70

Biomes O' Plenty (Forge):

• List of all blocks: https://github.com/Glitchfiend/BiomesOPlenty/blob/3d691903/src/main/java/biomesoplenty/init/ModBlocks.java
• Should erode examples: white_sand, mud, mud_bricks, origin_leaves
• Should block erosion examples: fir_wood, stripped_fir_log, redwood_log

Traverse (Forge):

• List of all blocks: https://github.com/MysticMods/Traverse/blob/33db1a8f/src/main/java/epicsquid/traverse/blocks/ModBlocks.java
• Should erode examples: FIR_LEAVES, RED_AUTUMNAL_LEAVES
• Should block erosion examples: FIR_LOG

Terrestria (Fabric)

• All blocks init: https://github.com/TerraformersMC/Terrestria/blob/453b1e67/src/main/java/com/terraformersmc/terrestria/init/TerrestriaBlocks.java
• Should erode examples: BASALT_SAND, BASALT_DIRT, BASALT_GRASS_BLOCK, BASALT_GRASS_PATH, BASALT_PODZOL, JAPANESE_MAPLE_SHRUB_LEAVES
• Should block erosion examples: REDWOOD, HEMLOCK, RUBBER, CYPRESS,WILLOW

Traverse (Fabric)

• All blocks init: https://github.com/TerraformersMC/Traverse/blob/ed4f1ad4/src/main/java/com/terraformersmc/traverse/block/TraverseBlocks.java
• Should erode examples: RED_AUTUMNAL_LEAVES, YELLOW_AUTUMNAL_LEAVES, FIR_LEAVES
• Should block erosion examples: STRIPPED_FIR_LOG, FIR_WOOD