With the release of the new header, the OnDemand-specific Confluence themes should have been migrated to the standard equivalents, but weren't.
The OnDemand Documentation Theme has regressed in functionality and appearance as a result. In particular, the sidebar toggle button doesn't appear, and the drag-and-drop resize doesn't work, which makes it difficult for users of this theme to hide and show the sidebar.
In the meantime you can fix it manually by switching your spaces to the plan Documentation Theme. You can change the theme by going to "Space Admin" in the Browse menu, choosing "Themes" in the "Look & Feel" section, then selecting "Documentation Theme".
You'll see a warning, "The theme you have selected does not include the Atlassian OnDemand navigation bar" but this warning is also a mistake that no longer applies with the new UI and will be removed. You can safely ignore it.
Once you've switched to the Documentation Theme, you will see the toggle button in the header and be able to drag the sidebar's right edge to resize it, just as before.