The only validly lost bitcoin is that sent to a burn address, like using the the usual OP-return method which means the output can never be spent, this is the only way the coin is lost in my opinion.
It's true that OP-return outputs are lost and can never be spent, but they are not the only coins that have provably been lost.
There were coins that were lost because miners didn't claim them when mining blocks and there were also coins that were lost due to the coinbase transactions having the same hashes.