In 1974, the Chinese government decided to make a syllabary for the Yi language, based on the symbols in Classic Yi. As with the Zhuang in the 1950s, it isn’t clear to me why if it was such a good idea to come up with a phonetic script for a minority language, they didn’t think it was an equally good idea for Chinese script.
The syllabary they came up with is stupid-huge. There are (depending on how you count) at least 756 characters, many more than any other syllabary. Part of the problem is that Yi has a very rich complement of sounds: 43 distinguishable consonant clusters, 8 distinguishable vowels, and four distinguishable tones in the reference dialect. (There are other dialects of Yi which use five.)
The astute reader will note that 43*8*5 is much greater than 756:. One of the tone’s glyphs are derived from another tone’s by decorating the glyph with a little hat. There are also some combination of vowels, consonant clusters, and tones that just don’t appear in the spoken language.
The astute reader will also note that 756 is much, much greater than 43+8+5, which they could have represented the sounds in if they had used diacritics to show the vowel and tone. I cannot find enough information on Classic Yi to tell, but it might be that the characters used in the Modern Yi syllabary are the Classic Yi characters for that syllable (similar to how Manyogana came from from Chinese characters), or derived from them (like Hiragana derived from Chinese characters).
Note that it is probably in part because of the existence of the syllabary that I can find so little information on Classic Yi. In addition to cutting the Yi people off from their written history, creating a syllabary also had the effect of cutting off interested bystanders from learning about the Classic Yi writing system.