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 xe2x86x92 Connections xe2x86x92 Data usage xe2x86x92 Data saver.
Data Saver status icon in Android Oreo update for Samsung