Has it? It’s pretty much been a gold-backed currency making gold the defacto reserve currency. Curious whether the USD has been the only fiat reserve currency.
Global currency reserves are currently ~60% USD, ~30% EUR and 9% GBP/JPY/CAD, USD use as a reserve currency has been trending down for decades now. As for previous reserves I would guess before the Euro the German Deutschmark and French Franc would both have been used for reserves instead.