Yeah, There was a reference in a paywalled article a year ago (https://www.theinformation.com/articles/openai-made-an-ai-br...): "Sutskever's breakthrough allowed OpenAI to overcome limitations on obtaining high-quality data to train new models, according to the person with knowledge, a major obstacle for developing next-generation models. The research involved using computer-generated, rather than real-world, data like text or images pulled from the internet to train new models."

I suspect most foundational models are now knowingly trained on at least some synthetic data.