\documentclass[20pt]{beamer}
\usepackage{beamerappendixnote}
\usepackage{lipsum}
\begin{document}
\begin{frame}{Main title}
  Main content \vfill
  \appxnote[allowframebreaks]{Long note}{
    \lipsum[2]
  }
\end{frame}
\printappxnotes
\end{document}