+1 to Adan89 comment here for past customers to be able to review products. It's tough to show the value of customer reviews to future potential customers if you don't have any reviews to show before toggling a feature that was jut released 😕.
One option that would be helpful is if once you toggle this feature on you have one email notification credit added to your account to be able to send an email to prior customers. That email could then direct those customers to a review page for their past orders. This would be beneficial to the seller site as well as Squarespace as a whole. Squarespace would get a boost in reviews on the platform, which would also help your sellers. Which in turn helps them sell more products = more sales on the Squarespace site and a happy seller interested in keeping their business on Squarespace rather than a competitor. @Karen_SQSP