Literary / historical register. Modern speech uses 仆人 (servant) or 助手 (assistant). 仆从 is preferred in historical fiction, court scenes, and political writing — especially the phrase 仆从国 (puppet state, vassal state).
history
In WWII Chinese sources, Nazi-allied states (Romania, Hungary, Bulgaria etc.) are routinely called 仆从国 — the 'satellite states' of the Axis.
