Does Reddit train AI on your posts?

Yes, and there is no opt-out setting. The only way out is not posting publicly:

• Reddit’s User Agreement: the license you grant "includes the right to use Your Content to train AI and machine learning models."
• There is no training toggle. Reddit’s Public Content Policy points to keeping content non-public instead: "If you prefer not to contribute content publicly, you have a number of options."
• The license runs one way: Reddit can license public content to others, while scraping it yourself for your own AI breaks its terms.