Dusting away dust from the old annals of Warband, here it says:
"With Warband key from Taleworlds, you could install independently. However for some time, you could convert your taleworlds CD-key to register on Steam for free. Thought that was 8 - 10 years ago. Many players did in fact keep Warband independent because of rumours that told you would have a better experience and stability if you refrained from booting tru Steam."