If I was a writer, I'd consider publishing my works under a license that explicitly bans AI training. What happens when those works inevitably get ingested by an LLM?

That clause of your license wouldn't be legally enforceable.

Your license can only operate with what copyright allows you to withhold initially.

A license that banned AI training cannot be enforced. It is meaningless. The same way you can't write a book with a license that readers are not allowed to write reviews of it.

Fair use cannot be restricted by license like that.

(You can engage in individual contacts with people, with terms like NDA's work, but those actually have to be signed and stuff, and you can't do it with public information like published writing.)