Possibly via keyboard shortcut see
#8936 (comment)

Zen Mode was removed as it was supposed to remove the space it needs in the header, as the header doesn't need much space anymore it is no longer needed. We might introduce a ZenMode later at some point which will be eg activated by pressing the key z which will fade out the left menu to have full width content. The left menu is not really needed anymore when using search

Not sure if we would show an icon again.

Another version will be to show only the icons and only a very small left menu when screen width is low. This would be a different issue though probably since complexity is higher.