This interesting article is from 2014, I wonder if there has been an update since then on lost bitcoins...
Rise of the Zombie BitcoinsPublished on June 22nd, 2014 by John W. Ratcliff
excerpt
....Will Power and Zombie Coins
For the purposes of this article a ‘zombie bitcoin’ is defined as all bitcoins associated with a public key address which has had no send transactions for over 18 months. Why 18 months? Because today bitcoins are worth about $600 apiece, and back in December of 2013 they were worth over $1,000 apiece. Anyone who owned bitcoins prior to 18 months ago would have acquired them at a maximum cost basis of probably about $30, and that is estimating very high. Much more likely most had a cost basis of under $10, and the average overall is well under $1. You would need to have incredible willpower to resist a 4,000% or even astronomically higher return on an investment. When we look at the history of these zombie coins, it turns out, that many people succumbed to their desire to cash out and they did indeed rise from the dead. As the price of bitcoin rises, the number of ‘zombie’ bitcoins outstanding declines over time. Each time there is a price rise, we can see it shake loose more zombies as they re-enter the active economy.
Over time a trend develops, fewer and fewer bicoins rise from the dead and those that remain appear ‘stuck’ and unchanged. At this time all of the outstanding early bitcoin mined blocks have almost completely flat-lined and are unchanged regardless of what happens with the price. It is extremely important to understand that, for the purposes of this analysis, bitcoins are considered ‘alive’ based on any spend transaction no matter how small. This means that if a person owns a thousand bitcoins that cost them $1 apiece and they then sell just one of those bitcoins; maybe to buy a coat at Overstock.com, or maybe they want to move it to a different wallet, then this action marks all one thousand of the bitcoins associated with that public key as being ‘alive’ even if they are the results of many outputs. For bitcoins to be considered ‘zombie’ coins, it means that they are 100% absolutely untouched in any way whatsoever for over 18 months.
While there may be plenty of people who ‘got in on bitcoin early’ over a year and a half ago when the price was a fraction of what it is today and these people are still holding out for even greater returns, one would expect many of them to at least cash out ‘a little bit’ for some fun. Realize that any ‘spend’ transaction demonstrates that the individual is in active control of that private key. We don’t care whether or not they are still sitting on a lot of potential profit, what we are trying to determine is which bitcoins are irrevocably lost forever and to do that involves determining whether or not anyone has access to the private key needed to move or spend those bitcoins.... much, much more
https://letstalkbitcoin.com/blog/post/rise-of-the-zombie-bitcoins