The only 100% proven destroyed coins are the one with OP_RETURN outputs.
Not sure how many BTC were destroyed in this way, but I remember an article saying, that there are about 4 BTC lost due to OP_RETURN outputs. Could be higher now, as it was an old article.

All other script outputs may be valid for someone to spend.

There is no way so far I know, to identify if any UTXO is never ever spendable in the future.

So I would not suggest to "steal" someone elses BTC.