The plugin isn’t abandoned. I don’t officially support the plugin on this forum, and the support ticketing system has changed a few times over the past year, so it’s possible we missed the ticket or were just busy. I don’t make any profit from the plugin, so it takes a backseat when I have other projects.
The website hasn’t been updated in a while, you are correct. It’s a free plugin, and it works.
Google doesn’t require image for the review snippet. Their validator says it is required, but it is not. Plenty of people receive star ratings in SERPs without them, and the test we did with including image didn’t change anything. It’s a recommendation, not a requirement. If it was a requirement, they wouldn’t accept the snippet and display it in SERPs.
That all being said, we do have plans to update it from the HTML markup format to ld+json format, which is less prone to issues with themes and such, and allows for easier customization on the website as the information being displayed doesn’t have to look weird to match the requirements of the HTML snippet. Don’t have a timeline on it yet, but we’re testing it out.