What is a more politically correct way to call something a "Red-Headed Step-Child"?

Most of these give general feelings which may (or may not) suggest a satisfactory term.
Some may fit as-is. One can hope :-).

• persona non grata.
• throwback
• standout
• developmental backwater (good for software)
• odd man out
• knife at a gun fight (getting desperate)
• black sheep of the family
• " ... C and all its bastard children" - Jerry Pournelle in full flight.
• evolutionary throwback.
• As useful as xxx on a yyy
• Like a xxx in yyy (eg snowball / hell, ...)

Consider:

• Pariah
  • An outcast.
  • A demographic group, species, or community that is generally despised.
  • Someone in exile.
  • A person who is rejected (from society or home).
• Outcast: One that has been excluded from a society or system, a pariah.
• Condemned: Having received a curse to be doomed to suffer eternally.
• Also-ran: A person or animal who competed in a race but did not win.
• Has-been: A person, especially one formerly popular or influential, who continues in their field after their popularity or effectiveness has peaked and is now in decline.
• Third wheel
  • A person or thing that serves no useful purpose.
  • An unwanted third party accompanying two people on a date.
• Spare wheel: An extra wheel held in reserve in most cars, to be used in an emergency.
• Fifth wheel: Anything superfluous or unnecessary.