documentclass{article}
usepackage[english]{babel}
usepackage{tikz}
usetikzlibrary{calc}
usepackage{lipsum}

begin{document}
tikz[overlay,remember picture]
{
    node at ($(current page.west)+(20.5,0)$) [rotate=90] {Hugetextcolor{gray}{Draft: today}};
}
lipsum
end{document}