Make the mobs drops that don't require custom enchantment code use modern, data pack friendly, JSON files.

For the record, we can't modify drops using CraftTweaker2-1.12-4.1.9 either.
It feels like the mod doesn't use the recently added loot tables.

No, it doesn't use these at all, that's the point. All the drops are hard-coded, just as in the 1.6.4 and 1.7.10 versions.