Out of curiosity, and IANAL, what is it in a GPL/copyleft license that would make it undesirable to train LLMs on projects with such license? Or are there stronger yet copyleft licenses that you had in mind?

(FWIW, and not directly related to my question, I always thought of GPL as the less (not more) restrictive license from the perspective of a user, because I could always ask for the source code and debug a problem on my own.)