> Something that surprised me is that [1] is of a minor key, but the solfege marks are for the relative major
Major and minor keys use the same shapes / solfege system. The major scale is "fa, so, la, fa, so, la, mi, fa", while the minor scale uses the same syllables but starts on la. This means that the shapes always represent the same intervals (i.e. pattern of whole and half steps) whether a song is in a major or minor. It's hard to describe but makes sense when singing.
One limitation is that the shapes are really only useful for singing, not for instrumental music. They also would not be meaningful for music that switches keys or that isn't tonal (i.e. isn't in a particular key at all). So they are a very good fit for what you might think of as "church music" (at least written before the 20th century), but for other settings they would be either meaningless or a hindrance.
That's one reason shape notation never caught on; another reason is that the notation was introduced by publishers of American sacred music and not by, say, music conservatories.
Is it possible that many of those uses predate the current zoning? There are numerous buildings in Chicago that would not be permitted by their current zoning.
I asked imagen 2 to generate a transparent product icon image, and it generated an actual grey and white square pattern as the background of the image... https://imgur.com/a/KA2yWHp