I just noticed that Google Customer Reviews Opt In stopped working in March, did the code change or something change on Shopify? I copied the information from Google Merchant Center and put it in the checkout settings. When I search the source page of an order confirmation page, there is no code. I put the following code in the checkout settings.
What makes you say it stopped working? If that's the code you're using, it won't work as the fields won't fill in correctly. You can follow this tutorial I've created to setup Google customer reviews in Shopify.
Run Google Shopping ads? Get the free definitive guide to Google Shopping for Shopify (no optin required): https://www.digitaldarts.com.au/google-shopping