Quick and dirty hack: smash your parbox

```
\documentclass{article}

\usepackage{mathtools}

\begin{document}

\begin{align*}
a &= b + c & &\smash{\parbox[t]{20mm}{this is a long bit of text}} \\
d &= e + f \\
\end{align*}

\end{document}
```

A different approach could be to use a table:

```
\documentclass{article}

\usepackage{tabularray}

\begin{document}

\begin{center}
\begin{tblr}{
  width=.6\textwidth,
  colspec={Q[r]@{\;}X[l]X[20mm]},
  column{1,2}={mode=dmath}
}
a &= b + c & \SetCell[r=2]{} this is a long bit of text \\
d &= e + f \\
\end{tblr}
\end{center}

\end{document}
```