It’s don’t know about the accuracy of the percentage. The 20% figure seemed echoed by Ivestopedia (see 20% of all BTC is lost, unrecoverable), based on other people’s studies. Chainanalysis went as far as stating, back in 2017, that almost 50% was, in fact, the figure.

Whatever the real number is, although a lot of it will come from the earlier days, it´s still seemingly not that easy, even nowadays, to ensure even passing them on (let alone not losing the keys, etc.).