Markwhen is alive and well, just not in this repository
Markwhen's components have been separated and live in other repositories under the mark-when organization.
The web app (the editor) is the only thing that isn't open source. Everything else (including the timeline itself and the vs code extension) is - check out the links below and the diagram of repositories image.

Markwhen is an interactive text-to-timeline tool. Write markdown-ish text and it gets converted into a nice looking cascading timeline.
Use the editor here.
This repo is for the view container, not the editor. The editor (markwhen.com) and VSCode extension are built on top of the view container.

The view container renders different views, like the timeline and the calendar. It is possible to create your own views using the view client library.