If custom or contrib code extends \Drupal\Core\Extension\ExtensionDiscovery and uses the \Drupal\Core\Extension\ExtensionDiscovery::$fileCache property, it will need to change before 12.0.0 is released.
If the code is writing a new entry to the cache it will need to add the property to the class and instantiate the file cache object during construction as \Drupal\Core\Extension\ExtensionDiscovery::$fileCache will no longer be set by the parent class.
If the code uses the cache entry written by \Drupal\Core\Extension\ExtensionDiscovery then this is no longer present and you need to optimise the cache miss path. If you are reading an info.yml file you can use the \Drupal\Core\Extension\ExtensionDiscovery::$infoParser property if it is available.
