Exclude headings from the table of contents when you insert two TOCs

Another approach:

documentclass{report}
usepackage{hyperref}

newififtocdetailed
makeatletter
defstartDetailedOnly{
    write@auxout{protect@writefile{toc}{protectcsname @startDetailedOnlyprotectendcsname}}
}

defendDetailedOnly{
    write@auxout{protect@writefile{toc}{protectcsname @endDetailedOnlyprotectendcsname}}
}

def@startDetailedOnly{
    unlessiftocdetailed
        xdefrestoreTocDepth{arabic{tocdepth}}
        setcounter{tocdepth}{-1}
    fi
}
def@endDetailedOnly{
    unlessiftocdetailed
        defsettocval##1{    
            setcounter{tocdepth}{##1}
        }
        expandaftersettocvalexpandafter{restoreTocDepth}
    fi
}

makeatother

begin{document}

setcounter{tocdepth}{1}
setcounter{secnumdepth}{3}

{expandafterdefcsname @starttocendcsname#1{InputIfFileExists{jobname.#1}{}{}}%
tableofcontents}

renewcommand*contentsname{Contents (detailed)}

setcounter{tocdepth}{3}

tocdetailedtrue
tableofcontents

chapter{XXX}

startDetailedOnly

chapter*{Conclusion}%
addcontentsline{toc}{chapter}{Conclusion}%to be excluded
section{YYY}
subsection{WER}
subsubsection{ABC}
subsubsection{DEF}
subsection{ZTR}
subsubsection{OIU}
subsubsection{OIZ}
subsubsection{POI}
section{YYY}
subsection{WER}
subsubsection{EDC}
subsubsection{RFV}
subsection{TGB}
subsubsection{ZHN}
subsubsection{OUH}
subsubsection{WEK}

endDetailedOnly

chapter{ZZZ}
chapter{AAA}

end{document}

When you use startDetailedOnly, all chapter's, section's and such until the following endDetailedOnly will be recorded, but only be shown in a TOC if you set tocdetailedtrue. This is done by changing tocdepth for non-detailed TOC's.