The 2019 change
Google stopped showing review rich results for reviews a business hosts about itself, under LocalBusiness or Organization. The reasoning was that self-rating is not independent evidence.
What still works
Product markup was never part of that restriction. Reviews attached to a Product remain eligible, along with Recipe, Book, Course and a few others.
What we output
Product schema with an AggregateRating, because that is the type that produces stars. It is on by default and there is a toggle.
Validation is not display
Correct markup makes a rich result possible. Google decides whether to show one, and it often does not.
Read the full version
The full explanation, with example JSON-LD, is in review schema markup explained.