Is the term "warfighter" new? I had never hears it until recently. Sounds like "soldier" would be the usual terminology.

It's more badass and cool, so it is used in public announcements now. All they have is rhetoric.

no, but it wasn't common. It's Hegseth's choice of words, just like Department of War isn't the name of the DoD but Hegseth puts it on his office door.