\documentclass{article}
\usepackage{fontspec}

\setmainfont{GaramondNo8}

\begin{document}

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{\addfontfeature{Numbers=OldStyle} 0 1 2 3 4 5 6 7 8 9}

\itshape
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{\addfontfeature{Numbers=OldStyle} 0 1 2 3 4 5 6 7 8 9}

\upshape\bfseries
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{\addfontfeature{Numbers=OldStyle} 0 1 2 3 4 5 6 7 8 9}

\bfseries\itshape
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{\addfontfeature{Numbers=OldStyle} 0 1 2 3 4 5 6 7 8 9}

\end{document}