Archiving Timelines is a great way to hold on to Timelines that you may not want to see every time you open the app.
Events contained in Archived Timelines are filtered out of the Special Lists by default. They are also excluded from the On This Day widget.
Tap and told on a Timeline row to show a context menu.
Select the Archive option. The Timeline will be moved to the Archive.
Tap on the Archive button in the lower left corner to navigate to the Archive.
All of your Archived Timelines live here.
You can Un-Archive a Timeline with the context menu.
You can also toggle the Archive status when editing a Timeline.