It's quite well explained here[1], which is linked from the Privacy section of their plan overview[2].

Basically individual accounts can opt out, while business and enterprise plans as well as API users can opt in.

You'd have to take their word, but that goes for anything in life.

[1]: https://help.openai.com/en/articles/5722486-how-your-data-is...

[2]: https://chatgpt.com/pricing/