Here’s what they’ve said:
“After November 1, mobile web pages that show an app install interstitial that hides a significant amount of content on the transition from the search result page will no longer be considered mobile-friendly. This does not affect other types of interstitials. As an alternative to app install interstitials, browsers [should] provide ways to promote an app that are more user-friendly.”
If you have mobile pages that have app interstitials on them, it would be a good idea to test them to see how much content they are blocking, and also to see whether your site has passed the mobile-friendly test.
Google have stated that they will only block app ad content, so using other ads that aren’t for apps is still fine to use.