Samsung Galaxy Note 8
My Question or Issue
Here's what I did to get rid of it on my Samsung Galaxy Note 8 running Android Oreo:
I don't know what the External applications notification is for. Depending on the purpose of the notification, it may or may not have a huge impact on the app. For this particular one, I can still play music on Spotify.
It doesn't seem to have any negative effects. However, just based on the name of the notification alone, it seems to have something to do with connecting to other applications, so that might get affected.
If you do follow this advice and start experiencing problems, you can always turn the notification back ON under Settings > Apps > Spotify > Notifications.
Hope this helps others.