(from the The Big, Fat Update ™️ commit)

Forge uses standard IForgeRegistry to register attributes(? actually, a GlobalEntityTypeAttributes class exists), while Fabric uses the FabricDefaultAttributeRegistry. Perhaps we should wrap this in some way

Don't know if this helps but this may be used as reference:
Common:
https://github.com/YummyLoop/Yummy/blob/0d8c780208f67613acbb5a04fbe73abeb2c1e578/common/src/main/net/examplemod/registry/EntityAttributeLink.kt
Fabric:
https://github.com/YummyLoop/Yummy/blob/0d8c780208f67613acbb5a04fbe73abeb2c1e578/fabric/src/main/net/examplemod/registry/fabric/EntityAttributeLinkImpl.kt
Forge (deprecated):
https://github.com/YummyLoop/Yummy/blob/0d8c780208f67613acbb5a04fbe73abeb2c1e578/forge/src/main/net/examplemod/registry/forge/EntityAttributeLinkImpl.kt
Forge:
https://github.com/YummyLoop/Yummy/blob/8f550d46e1dbc11d45ab38fc8a403e7dd13b881b/forge/src/main/net/examplemod/registry/forge/EntityAttributeLinkImpl.kt