If take a random photo from the Internet, then on the X-ray of the luggage can see the well-known $5 wrench, on which the factory engraved text on the handle is slightly visible, which allows us to assume that, in theory, the text from the metal backup can be seen in this way.


But there are nuances. For example, if the metal backup is not on one plate, but several (like round washers fastened together), but I assume that the text will not be read because all the symbols on one plane will "merge" into one mess of symbols (create "noise" in the image). Also, if the washers are placed on their edge, that will also not allow reading the text from them. I want to say that in order to see your seed-phrase under the X-ray image, several factors must coincide and, if desired, the chances of this can be minimized.

After all, x-rays are used to identify prohibited items, not to read fine print on small items. If you haven't put 12\24 words on the handle of a gun or the blade of a knife, then it's unlikely that your luggage will be of interest to anyone (in the case of an airport).