I’d like to leave a rating or review for an author. I think it’s very important that others hear my experience, how can I share this if the original item has been removed?
Not sure what you’re asking. If there is no item to review then what’s the point of leaving one and to who is this useful? 
I’d like to leave a review and/or rating for the author more importantly, not the product.
No way to do so I’m afraid. I’m curious why you’d want to though…Envato sells products and services, so you should only be able to rate products and services, not people. I’m going to assume you want to rate the service an individual has provided, and not the actual individual? You’d normally be able to as part of the review on the item you purchased, but if the item has been removed then there’s no solution unfortunately.