I have a deprecated function and ask ChatGPT what I should use instead, ChatGPT responds by inventing a random non existent function. I tell it that the function doesn't exist, it tries again with another non existent function.

Oddly speaking that sounds like a very simple language level failure, i.e. the tool generates text that matches the shape of the answer but not its details. I am not far enough into this ChatGPT religion to gaslight myself over outright lies like Elon Musk fanboys seem to enjoy doing.