What is this triangle icon [Samsung Galaxy S9+]? It appeared recently. And I can't even imagine what is it. 

 

Screenshot (click image for larger variant)
That's a Data saver icon.


  The icon is different from the data saver icon in stock Android.
  Samsung started to use this data saver icon in Android Oreo update as
  explained in this guide


This Galaxy S9 status icon appears  if you enable data saver in Settings → Connections → Data usage →  Data saver.



Data Saver status icon in Android Oreo update for Samsung