We don't know if the 21 million will be circulated at the end of the mining, remember Nakamoto holds 1 million Bitcoin and we do not know if this amount of Bitcoin will ever move and there are those who lost from owner's mismanagement, there should be a study on the full circulation of Bitcoin or some sort of inventory.
There are some problems with doing something like that, I understand the desire to know exactly how many coins are actually available and how many of them are lost, but even if you could track all the coin movements during the last year this will not tell you if the coins that have not moved during that time frame are lost, there are many long term holders that are storing their coins in paper or hardware wallets that have no intention of moving them in years or decades, so the figure you could get will be inexact at best.