Source: Discord

I kind of hate to bring this up now, after all of the classic PRs have been merged, but I don't think this works as a single addon for all three versions of the game. The different versions have different drop criteria for the same item.
For example: The Dark Whelpling.
Retail, it's dropped by eight NPCs
Cataclysm, it's dropped by six NPCs
Classic, it's dropped by two.
Some of the NPCs that drop the pet in Cataclysm also exist in Classic Vanilla, but the vanilla NPC doesn't drop it.

Someone would have to go through the database and identify what alterations need to be applied. We should have the infrastructure to test variations now, but I definitely don't have the time and energy to review every item myself. Alternatively, wait for bug reports (not ideal).

Items that have been reported as broken on Classic:

• Cat Carrier (Black Tabby): Zone drop on Retail, rare drop in Classic (Ambermill Spellscribe)
• Azure Whelpling (Winterspring)
• Tiny Crimson Whelpling
• Sprite Darter Egg (Grimtotem Marauders)
• Razzashi Hatchling
• Hyacinth Macaw

See also:

• Not tracking zone drops in Cata Classic (WowAce tickets)
• #834
• Various Discord posts which I am too lazy to find and add here right now (use the search function if needed)