Up to 25% lost for good according to this source: https://letstalkbitcoin.com/blog/post/rise-of-the-zombie-bitcoins https://letstalkbitcoin.com/blog/post/rise-o

Up to 25% lost for good according to this source: https://letstalkbitcoin.com/blog/post/rise-of-the-zombie-bitcoins https://letstalkbitcoin.com/blog/post/rise-of-the-zombie-bit... & https://seekingalpha.com/article/4082979-many-bitcoins-lost-forever https://seekingalpha.com/article/4082979-many-bitcoins-lost-... Also many wallets are simply unspendable due to transaction costs.