upright italics (math mode)?

CMU isn't Knuth's original Computer Modern font but a newer extended Unicode set using OpenType font format.

LaTeX only provides the simple named macros for the combinations that are most likely used but all of the original Computer Modern collection can be accessed by specifying appropriate font family or shapes, for example:

\documentclass{article}

\begin{document}



{\fontshape{ui}\selectfont abcdefg  upright italic}

{\bfseries  abcdefg bold extended}


{\fontseries{b}\selectfont  abcdefg bold non-extended}

{\fontfamily{cmfib}\selectfont  abcdefg fibonacci}

{\fontfamily{cmdh}\selectfont  abcdefg dunhill}


{\ttfamily  abcdefg non-variable typewriter}

{\fontfamily{cmvtt}\selectfont  abcdefg variable typewriter}


{\fontfamily{cmvtt}\itshape  abcdefg variable typewriter italic}





\end{document}

Are there packages which make [upright italics] accessible?

You could load the cfr-lm package, which provides the \uishape switch and the \textui macro. (ui is short of 'upright italic'.)

\documentclass{article}
\usepackage{cfr-lm}
\begin{document}
$aaa,\ \mathit{aaa},\ \textit{aaa},\ \textui{aaa}$

$fff,\ \mathit{f{}f{}f},\ \textit{f{}f{}f},\ \textui{f{}f{}f}$
\end{document}

The CTAN/TeX Live/MiKTeX package cm-unicode also contains OpenType fonts that can be used with LuaLaTeX or XeLaTeX, e.g.:

\documentclass{article}
\usepackage{lipsum}
\usepackage{fontspec}

\newfontface\cmunui{CMU Serif Upright Italic}

\begin{document}
  \cmunui
  \lipsum[1]
\end{document}