Facebook has announced some flexibility in their policy to refuse ad images with more than 20% text. Thanks to @Margaret_Marie for alerting us to this article.
Previously, if 20% of an ad image’s area was text, it wouldn’t be approved to run on Facebook, Instagram or the Audience Network. While ads with minimal text are still preferred, we’ve adopted a new system that allows you to run ads that would’ve been rejected under our old policy. With our new system, ads with higher amounts of text will receive less or no delivery at all (unless we apply an exception). Use the text overlay tool to determine if your ad’s reach may be reduced by too much text on the ad image before you run your ad.
When you use images, try to include as little text as possible on the image itself. For any text you use in your ad, we recommend that you include it in the post, rather than the image. Our research has shown that people prefer to see ads with minimal image text.
The article includes examples of overlaying text in ads.