Unicode restriction

In src/backend/utils/mb/wchar.c there is a check to exclude Unicode
characters above 0x10000.  I can't see anything to explain this
restriction, except possibly this in the release notes for 7.2:

        Reject invalid multibyte character sequences (Tatsuo)

It does not explain why part of the Unicode character range is invalid. 
There is a Debian bug report from someone whose client is trying to
store characters in the excluded range.  What would be needed to enable
support for it?

-- 
Oliver Elphick                                          olly@lfix.co.uk
Isle of Wight                              http://www.lfix.co.uk/oliver
GPG: 1024D/A54310EA  92C8 39E7 280E 3631 3F0E  1EC0 5664 7A2F A543 10EA
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     "Love is patient, love is kind. It does not envy, it
      does not boast, it is not proud. It is not rude, it is
      not self seeking, it is not easily angered, it keeps
      no record of wrongs. Love does not delight in evil but
      rejoices with the truth. It always protects, always
      trusts, always hopes, always perseveres." 
                                I Corinthians 13:4-7