Subscription database ER model design
First, if the idea is that users subscribe to products, so that they can hear about new reviews on that product, then you definitely want the (many-to-many) relation to be between user and product, not user and reviewproduct. You're not subscribing to a review, you're subscribing to a product.

Regarding how do you track the new reviews: My first thought would be to keep a timestamp on the user-product subscription entity. When you want to know if there are new reviews for a product, you just query for all reviews created after the timestamp, and then you update the timestamp (or you update it whenever the user views them, or whatever your requirements are exactly).

(Side note: Not sure why you're calling reviews “reviewproduct”. I'd just call it “review”, or if you need to differentiate it from some other kind of review, maybe “productreview”.)