Admob wrote me a email:
Update the Google Mobile Ads SDK for iOS to ensure compatibility with the App Store
In December last year, Apple announced that from April 2020 on the App Store it will not be possible to publish applications that use UIWebView, and from December 2020 - corresponding updates for applications.
All versions of the GMA SDK, starting with 7.55.0 (dated February 4, 2020), by default use WKWebview, not UIWebView.
To ensure compliance with the requirements of the App Store, follow these steps:
1) Update the GMA SDK to version 7.55.0 or later as soon as possible and apply it to the application.
2) If you use Open Bidding or in-app mediation, check out the information below. It lists the minimum SDK versions of third-party ad networks that support WKWebView. Also make sure that the adapter versions match the SDK.
List of minimum new SDK versions of third-party ad networks that use WKWebView