Flash screen for Slack alerts
From an email with Slack support:


  The MacOS flash screen setting is based on the system default "alert"
  sound. If an application uses the system default "alert" sound, the
  screen will flash. Since Slack never plays the system default sound,
  we'll never flash the screen.
  
  For now, I'm afraid this means there is no way to flash the screen
  when Slack receives a notification. That said, I've let the team know
  that this is something you'd like to see in the future.


This was written on the 16th May 2019. Slack v3.3.8