\documentclass{moderncv}
\usepackage[firstyear=2000,lastyear=2025]{moderntimeline}
\moderncvstyle{classic}
\moderncvcolor{blue}
\name{John}{Doe}
\begin{document}
\makecvtitle
\tllabelcventry{2005}{2020}{end}{Scribus Student}{GNU School}{The Internet}{}{Tried Scribus for a short while}
\end{document}
Result

Expected result: No 2020 in the timeline, but only "end".
It works by doing
\tllabelcventry{2005}{2020}{end///}{Scribus Student}{GNU School}{The Internet}{}{Tried Scribus for a short while}

Thanks @exaexa for the "workaround".
Result
Expected result: No 2020 in the timeline, but only "end".
It works by doing
Thanks @exaexa for the "workaround".