The Jargon File (version 4.4.7, 29 Dec 2003):

backward combatability
 /bak'w@rd k@m?bat'@?bil'@?tee/, n.

    [CMU, Tektronix: from backward compatibility] A property of hardware or
    software revisions in which previous protocols, formats, layouts, etc. are
    irrevocably discarded in favor of ?new and improved? protocols, formats,
    and layouts, leaving the previous ones not merely deprecated but actively
    defeated. (Too often, the old and new versions cannot definitively be
    distinguished, such that lingering instances of the previous ones yield
    crashes or other infelicitous effects, as opposed to a simple ?version
    mismatch? message.) A backwards compatible change, on the other hand,
    allows old versions to coexist without crashes or error messages, but too
    many major changes incorporating elaborate backwards compatibility
    processing can lead to extreme software bloat. See also flag day.


The Free On-line Dictionary of Computing (30 December 2018):

backward combatability

    /bak'w*d k*m-bat'*-bil'*-tee/ (Play on "backward
   compatibility") A property of hardware or software revisions
   in which previous protocols, formats, layouts, etc. are
   irrevocably discarded in favour of "new and improved"
   protocols, formats and layouts, leaving the previous ones not
   merely deprecated but actively defeated.  (Too often, the old
   and new versions cannot definitively be distinguished, such
   that lingering instances of the previous ones yield crashes or
   other infelicitous effects, as opposed to a simple "version
   mismatch" message.)  A backward compatible change, on the
   other hand, allows old versions to coexist without crashes or
   error messages, but too many major changes incorporating
   elaborate backward compatibility processing can lead to
   extreme software bloat.

   See also flag day.

   [Jargon File]

   (2003-06-23)