Please clarify assets usage rules. Can we use assets outside of listed resources - Kitbash 3D, Game textures?

Can we use CC0 assets available on itch.io or opengameart.org?

Yes, you can use assets outside of the listed resources and CCO assets as long as you have the rights to use them. When submitting your game entry you will be required to list all of the assets you used that were made before the game jam.