When Google removes malicious apps from their Play Store, are users notified?
By default, no. This would require Google to sniff the apps in your phone, in which they should ask your permission to do so.

However, there is this:

Google Play Protect


  How Google Play Protect works
  
  Google Play Protect checks apps when you install them. It also
  periodically scans your device. If it finds a potentially harmful app,
  it might:
  
  Warn you. If an app is detected that may be harmful to your device,
  you’ll get a notification. To remove the app, tap Uninstall on the
  notification. Google Play Protect might also disable the app to keep
  it from running until you uninstall it.
  
  Remove the app automatically.
  In some cases, if a harmful app has been detected, you may get a
  notification saying the app was removed.