"Improve the products and services of our family of companies and offer new features. For example, using information to train, develop and improve our technology such as our machine learning models, regardless of when Pins were posted. This includes, for example, Pinterest's body type technology, which is trained on images in Pins posted to our Services."
Pinterest claims the right to use all user-uploaded content (Pins, photos, images) to train its AI models indefinitely, with the phrase 'regardless of when Pins were posted' suggesting a perpetual license over user content for this purpose.