I remember listening to a thing on 5 Live about this ages ago pre covid - I think it was Martin Lewis who was on as a guest, and he said something along the lines of shops could choose to only accept Monopoly money or Pokemon cards if they so chose and it'd be perfectly legal. The "legal tender" thing is utter horse shit apart from, as you say, when it's related to a debt.
The guy above who was on about "coins of the realm" probably also believes you can get out of paying council tax and parking fines by quoting some obscure line from the Magna Carta because some nutter on YouTube said so...