It's not unprecedented, but I'm not sure if the reserve currency holder has ever done it that honestly. Technically, the US defaulted in 1933 and 1972, but that's not how we tend to look at it.

In 1933, the dollar was defined in terms of gold, so $20 of loans meant 1oz of gold. When they officially devalued the dollar, debts were kept in their dollar amounts rather than their gold amounts. All debt holders took enormous haircuts.

In 1972, foreign central bank deposits at the Fed were defined in terms of gold. So, when Nixon uncoupled the currency from gold that was another major default.