I don’t know why everyone is suggesting complicated means to destroy the steel plate. Acid is expensive and hard to come by and how will you melt it at 3000C in your home?
All you need to do is take a drill and make a few holes in it and you are done. If the drill breaks it means you got a cheap bit. Buy one of those $5 bits and it will drill thru steel like butter pretty much.
You also don’t need to drill all the way, just enough to destroy what is written on the main surface.
A drill or a pair of metal cutting pliers can easily ruin the seed phrase written on that steel plate. I find that protecting them is really a very difficult task while destroying them is not a difficult problem. Also, if we can engrave words on the steel plate, why don't we try changing some of the words on it? That way, if someone picks up those steel plates they won't have our real seed phrase.
We come up with thousands of ways to keep our seed phrases as safe as possible, and now we find ways to destroy them, which is interesting

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