With reference to your query about countries which do not demonetise their old currency there was a BBC story about Germany recently. Although the German Mark is not legal tender it still can be changed to euros.

Another point that occurred to me is what amounts of coins are legal tender. In this country copper coins – one and two pence - are only legal tender up to twenty pence worth. So you could not pay a tax bill in pennies. I do not know if you have an equivalent law in the US

The Bank of England will still redeem any of their obsolete notes, even though they have lost their legal tender status in commerce. In practice, many London banks will also exchange obsolete BoE notes that can no longer be used in trade.

No one-yen note of the Bank of Japan has ever been demonetized. That is why one-yen notes illustrated in books have specimen markings imposed on them, no matter their age. Pre-1946 Bank of Japan notes are no longer exchangeable, and no specimen markings are needed on them when published.

Most writings on banknotes by bank tellers are numerals indicating the amount of money in the stack, when less than a full strap is involved (50 or 100 notes). They write the numbers on the watermark window, a convenient blank space that begs to be "scribbled" on.