It's probably not. Exchange hot and cold wallets hold insane amounts, but most of that money doesn't belong to them, it's their clients'.
Or maybe better. In the first days of bitcoin nobody cared about a lost key. A few thousand bitcoin on an account nobody has access to? These bitcoin shouldn’t count, but it’s hard to count them. Maybe declare all bitcoins that didn’t move since eight years as lost?
And this is a stupid metric too. I have utxos that old that will be unspent for another 5y+
I don’t care about Bcash and other worthless forks and I could tell it was an attack to unmask old utxos.
Coinbase cold storage holds billions of dollars for millions of people. Do you understand how bad this metric is now?