Up to 25% lost for good according to this source: https://letstalkbitcoin.com/blog/post/rise-of-the-zombie-bit... & https://seekingalpha.com/article/4082979-many-bitcoins-lost-...

Also many wallets are simply unspendable due to transaction costs.

This is interesting but my understanding of blockchain is limited so: Aren't all transactions 'encoded' in the blockchain? If so why can't the coins be found? If not, what does the blockchain "record"?

The coins are there but since the private key for the wallet that is assigned ownership of them is lost they're effectively gone.

They could in theory be 'reclaimed' with a majority of mining power but it's hard to tell if coins are lost or their owners are just idle.

If the private keys corresponding to the coins are lost then the coins are, for all intents and purposes, lost. They can't be sold, moved around, etc.

Very small transaction outputs can't be used in practice because they're less than the required transaction fee.