At the top of the Firebase console you now see this:
"Please upgrade to the generally available Firebase Crashlytics SDK to continue getting crash reports in the Firebase console past November 15, 2020."

Also (https://firebase.google.com/docs/crashlytics/get-started?authuser=0&platform=ios):
Note: The Fabric SDK is now deprecated and will continue reporting your app's crashes until November 15, 2020. On this date, the Fabric SDK and old versions of the Firebase Crashlytics SDK will stop sending crashes for your app. To continue getting crash reports in the Firebase console, make sure you upgrade to the Firebase Crashlytics SDK versions 17.0.0+ for Android, 4.0.0+ for iOS, and 6.15.0+ for Unity.

This involves basically to use Google's version of Crashlytics instead of the old Fabric version. Read up on the upgrade docs or other docs to do this work:
https://firebase.google.com/docs/crashlytics/upgrade-sdk?authuser=0&platform=ios