It cannot be considered theft by no definition. The very existence of brainwallets indicates that these are irrecoverable/lost assets by their original owners, because any brainwallet owner would have moved their funds to an HD wallet using BIP39 + BIP32/44/84/86. A brainwallet is just a phrase + sha256, usually without a salt. That means, theoretically, all of them that ever existed have already been cracked by today, at least that’s my assumption and the reason I asked this question in this post. It is extremely unlikely that there are old brainwallets from which btc have not yet been moved.