unicode.org/reports/tr15/#Forbidding_Characters having the rule "An example of such a post composition version exclusion is U+2ADC (⫝̸) FORKING. To date, that one character, encoded in Unicode 3.2, is the only character added to the list of composition exclusions based on the criterion of its decomposition mapping containing a prior-encoded character." which translates as "we really messed up with this character so we are adding a special rule to make it hard to use"