It does not seem that multicols environment can cross across pages. Is there an alternative to create multiple columns that are filled automatically (i.e., we just insert the items and the environment fills columns using its own algorithms) with the contents but can break frames if necessary?
\documentclass[aspectratio=169]{beamer}
\usepackage{blindtext}
\usepackage{multicol}
\begin{document}
\begin{frame}[allowframebreaks=0.7]{Long Lists}
\begin{multicols}{2}
\blindlistlist[3]{enumerate}
\blindlistlist[3]{enumerate}
\blindlistlist[3]{enumerate}
\end{multicols}
\end{frame}
\end{document}

allowframebreaks: "The use of this option is evil." The content of each slide should be carefully planed and not the result of some automatic column or page breaking. – samcarter_is_at_topanswers.xyz Mar 19 '19 at 23:32