one could possibly argue that this should be have been a standard decomposition rather than a compatibility decomposition.
I might agree with someone making this argument, but it is not the argument I'm making or have made in this thread.
"7/8" isn't a grapheme, much less the same one as "⅞".
(where the "/" above is shorthand for U+2044 FRACTION SLASH)
My understanding of the intent of the Unicode FRACTION SLASH character is that it's intended to draw exactly such an equivalency. Your web browser agrees with this interpretation, rendering the sequences identically, as your footnote explains. Any use of a slash-like character to separate 7 and 8 with other meanings (like as part of a date) would have to use a slash other than U+2044. What exactly is the purpose of having a slash specifically earmarked for fractions if not to indicate that the string containing it does in fact represent a fraction?
Nonetheless, as you note, Unicode does make clear that this decomposition is a compatibility one rather than a standard one, so the Perl library correctly reflects this.
