CryptoKitties / KittyHats was one of the first examples of extensibility on Ethereum. You can see the implementation, the Solidity contract, here: The accessory token is attached to a kitty by storing the kitty ID in the token, i.e. it is a bottom-up approach.
This is a really good blog post explaining the bottom-up and top-down approaches (it is even using KittyHats as an example): Top-Down and Bottom-Up Composables, What’s the Difference and Which One Should You Use? | by Nick Mudge | HackerNoon.com | Medium.
ERC-998: Composable Non-Fungible Token specifies both approaches to support backwards-compatibility, i.e. existing contracts (which are immutable on Ethereum).
If we add this proposed extensibility proposal to Cadence, existing contracts would automatically gain support without any changes required by contract authors, which would obsolete the bottom-up approach, as the top-down approach could then always be used.