In my opinion, forgetting a private key does not make the BTC lost. Its still there on the timechain ready for anyone to move it if they guess/remember the private key.

However, there are some BTC which are provably lost. For example:

When a miner mines a block, the protocol allows that miner to reward themselves newly created BTC. Sometimes a miner does not claim the full amount that the protocol would allow them. Suppose a miner could create 50 BTC, but they messed up their block template and only gave themselves 5 BTC. Those missing 45 BTC are gone for good, permanently reducing max the supply of BTC that will ever exist.

Anyone can make a chart and claim the "lost" BTC is anything they want (without sources). I wish more charts would distinguish "provably burned coins" vs "maybe lost coins".