discardpages{-2,-1}

keeppages{-3,-1}

documentclass{article}

usepackage{lipsum,atbegshi,etoolbox}
usepackage{lastpage,refcount}

makeatletter
newcommand{discardpages}[1]{% discardpages{<csv list>}
  xdefdiscard@pages{#1}% Store pages to discard
  AtBeginShipout{% At shipout, decide whether to discard page/not
    renewcommand*{do}[1]{% How to handle each page entry in csv list
      ifnum##1<0
        ifnumvalue{page}=numexprgetpagerefnumber{LastPage}+##1+1relax
          AtBeginShipoutDiscard% Discard page/not
          gdefdo####1{}% Do nothing further
        fi
      else% ##1 >= 0
        ifnumvalue{page}=##1relax%
          AtBeginShipoutDiscard% Discard page/not
          gdefdo####1{}% Do nothing further
        fi
      fi
    }%
    expandafterdocsvlistexpandafter{discard@pages}% Process list of pages to discard
  }%
}

newififkeeppage
newcommand{keeppages}[1]{% keeppages{<csv list>}
  xdefkeep@pages{#1}% Store pages to keep
  AtBeginShipout{% At shipout, decide whether to discard page/not
    keeppagefalse%
    renewcommand*{do}[1]{% How to handle each page entry in csv list
      ifnum##1<0
        ifnumvalue{page}=numexprgetpagerefnumber{LastPage}+##1+1relax
          keeppagetrue% Page should be kept
          gdefdo####1{}% Do nothing further
        fi
      else% ##1 >= 0
        ifnumvalue{page}=##1relax%
          keeppagetrue% Page should be kept
          gdefdo####1{}% Do nothing further
        fi
      fi
    }%
    expandafterdocsvlistexpandafter{keep@pages}% Process list of pages to keep
    ifkeeppageelseAtBeginShipoutDiscardfi% Discard page/not
  }%
}
makeatother

%discardpages{-2,-1}
keeppages{-3,-1}% Keep these pages.

begin{document}

sloppylipsum[1-50]lipsum[1-50]lipsum[1-50]lipsum[1-50]lipsum[1-50]

end{document}