system - 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)}

Answer

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