Item drops from iron golems and cats aren't affected by looting enchantment. Here's the list of mob drops without looting enchantment

• Iron Golem drops 3-5 iron ingot and 0-2 poppies
• Cat drops 0-2 string

Steps to Reproduce:

1. Make sure to enchant your sword with Looting 3, using /enchant @s looting 3
2. Summon Iron golem and cats
3. Kill it using the sword enchanted with looting and observe the loot that it drops upon death

Observed Results:

• Killing Cat doesn't increase string drops
• Killing Iron Golem doesn't increase Iron Ingot and Poppy drops

Expected Results:

Both cat and iron golem drops would be affected by the looting enchantment.

Code Analysis:

Code analysis by OcelotOnesie can be found in this comment.