I'd pay for a cheaper app version of this that I could run on an old no-SIM phone.

As the page points out (top-right corner), this thing is the physical accessory to an existing phone app: https://busy.app/

Though, mind you, said app doesn't perform exactly the same function as the bar itself; "Busy App" seems to be more of a distraction-blocking pomodoro timer for your own phone, rather than a digital "busy sign" for you to point at the rest of the room.

IMHO, presuming a desk implies a PC (or laptop dock) with at least one spare USB port — the simplest thing here wouldn't even be an old phone; it'd just be a red USB light (think a red siren, but without the moving/flashing) that you could set on top of your desk/monitor/cubicle wall, and control with a little menu bar widget (perhaps automatically, by heuristic.)