Vanillabean adds much more content to Cats in the form of Cat bartering!
A TL;DR is at the bottom of this page.
If a thrown item is within 1.2 blocks of a tamed, sitting Cat, it may use it to barter, eating it in exchange for future rewards. Only fish-type items are bartered with. Fish can still be fed to Cats normally by right-clicking.
A Cat can only barter once every 10 minutes. If a Cat can't barter at the moment, it will display smoke particles.
Bartering with a Cat will increase its "favor," a hidden value that determines how friendly and loyal your Cat is to you. Different fish items will increase favor by different values. Typically, rarer fish increase favor significantly more. You can read about favor values in the collapsible menu below.
Cats do lose favor over time, but at a very a slow rate (1 favor every 30 seconds). If a Cat is currently on bartering cooldown, it will not lose favor. So, keep giving Cats fish to stop their favor dropping! Additionally, Cats which have gone above 150 favor will never fall below it. This means some benefits from Cat bartering can be kept permanently.
Owning Cats with higher favor can give significant rewards and benefits. See the collapsible menu below.
Note: Many Cat bartering gifts can be received as morning gifts. This is a mechanic in default Minecraft which Vanillabean expands upon.
See https://minecraft.wiki/w/Cat#Gifts
Cod (1)
Salmon (1)
Trout (1)
Codsidian (1)
Obsidibass (1)
Snapper (2)
Bass (2)
Void Wisp (2)
Perch (3)
Carp (3)
Pufferfish (3)
Tropical Fish (4)
Strider Guppy (10)
Sunken Salmon (10)
Magma Carpa (10)
Soulphisher (10)
Haddock (50)
Tuna (50)
Alpha Cod (50)
Lobster (60)
Eel (60)
Goldfish (60)
Anglerfish (60)
Marlin (80)
Mahi-Mahi (80)
Lava Marlin (300)
Owning Cats with higher favor can lead to some great rewards and benefits. Benefits that require 151 favor or more cannot be kept forever, but all others can. If a Cat reaches 150 favor, it will never drop below.
Better Gifts (4+ favor)
This Cat is more likely to give a Phantom Membrane as a morning gift, and also has a chance of giving a Nautilus Shell, Gold Ingot, Gunpowder, Cake, Bottle o' Enchanting, or Block of Copper.
Fish Trap Boosting (90+ favor)
Nearby Fish Traps catch fish more often.
Regeneration (150+ favor)
Every 10 seconds, all players within 20 blocks receive Regeneration for 15 seconds, and the Cat displays a heart particle.
Valuable Gifts (165+ favor)
This Cat has a very high chance of giving (as a morning gift,) a Diamond, Block of Gold, Block of Emerald, Spawner Fragment, Enchanted Book (Mending), Enchanted Book (Midas Touch), or Enchanted Book (Agility).
Health Boost (175+ favor)
Every 10 seconds, all players within 20 blocks receive Health Boost II for 15 seconds.
Fishing Boost (175+ favor)
Fishing Bobbers within 20 blocks have a +100% chance of catching rare fish, and will display shiny particles.
(Effectively a text summary)
Cats can now be fed fish-type items by dropping them nearby, increasing their hidden "favor" value. Rarer fish tend to increase favor values by significantly higher amounts.
Having higher favor results in Cats giving some amazing benefits, rewards, and buffs (see the collapsible menu above).
Favor decreases slowly over time, but can never drop below 150 once that threshold has been passed.