That is an interesting question... and I happened to read an article about that last week.

According to Coinmetrics on their article (https://coinmetrics.substack.com/p/coin-metrics-state-of-the-network-d2e), by block 600,000 there were at least 1,500,000 coins lost... and that can happen for several reasons, in different magnitudes: you have the 50 genesis coins, stolen or zombie coins, duplicate transactions (yes, duplicate), unclaimed rewards and so on...

It is hard to say for sure, of course. We don't know if the zombie coins are just being strongly hodled or if they are in fact lost.

One way or another... interesting read. Highly recommend!