Catnip can be consumed by all Nekos as much as they want. It's current implementation mostly just for role-play purposes. To make catnip you take seeds and just rename them to "catnip". This is not case sensitive.
Currently catnip only applies temporary hunger, slowness, weakness, and Regen. Not something really useful in survival, and mostly just there for people role-playing with NekoC. In the future I will add in more ways to have it better in a survival setting. However it shouldn't break survival in any way currently regardless.
Catnip can be disabled by a config setting, being "Catnip: false", this option has been defaulted to true in the config, but will not automatically get added to your previous config. Please add it to avoid future issues.
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I also fixed a bug where Hiss would work for normal players.