It started life as a cashew in KDE 4 and morphed into a hamburger in Plasma. Now it is gone. What was officially known as the "Desktop toolbox", the menu in the upper right (sometimes upper left) corner of your desktop has been removed.
Instead, long clicking (long press on touch-screens), or right clicking and choosing "Customize layout" brings up a control panel. Use it to add widgets, manage workspaces and configure the desktop. Learn about more changes here: