To calculate realized Cap, you can access data from Glassnodes
https://glassnode.com/ about Coins that haven't moved for a long time.
Coins that have not moved in 15 years are not definitely coins that are lost!
Coins that have moved yesterday might be coins that have been lost!
Nobody in this world would be able to approximate this, not by a very large margin that is proven day after day that is wrong, besides, it doesn't even matter. Market cap and realized market cap are some indicators that mean little, a far more reliable indicator would be to gather all data from major exchanges that have existed for the last decade and look at the inflows and outflows, which would be far more interesting as it would actually tell you how much was invested compared to the current value of those investments.
What we usually care about is the total supply and how many Bitcoins are left to mine. That’s the kind of data we use to analyze the market and make decisions.
Who is this "we" because I've never heard of someone trading based on how many Bitcoins are left to be mined!