> Cool but wouldn’t you forget how to decode it after some time?
Not necessarily.
First of all there isn't much decoding to be done: if one follows the recommendation, and one uses the hex input variant as the password (or secret), then decoding is as simple as just writing the number is binary base. For example this password `d74ae47dc6f599d3f9cb847bd77d6b7c` can be recovered by simply starting a Python interpreter and writing `"%032x" % b1101011101001010...`
Moreover, by just providing the card to a computer science graduate / enthusiast, his first hunch would be to just try to convert the bits into a number and use that (either base10 or base16), perhaps after rotating / flipping the card (if he didn't knew about the alignment). (This can easily be deduced because there are 2 groups of 8x8 bits.)
(Alternatively, if one scratches on the card the URL `purl.org/999/1`, provided Archive.org, which currently hosts PURL.org, doesn't delete the links like Google just did with `goo.gl`. I intend to update the redirect from this PURL to a page hosted by Archive.org.)
> I.e. wouldn’t engraving regular characters be simpler?
The first, and perhaps greatest issue, is that not many people have access to engraving hardware. One could go to a shop and have the characters engraved, but then they wouldn't be secret.
So, the main purpose is this: how could one with minimal access to power or precision tools, create the sturdiest physical backup of a small piece of information.
Thus this proposal: with a sheet of metal, a nail, and a hammer, one could just encode the 128 bits by banging with the hammer on the nail through the sheet of metal. :)