This new button is available in Canary.
ICYMI: Google is testing a side panel that extends to the top of the browser window, but this new panel design will coexist with, and not replace, the current layout of Chrome's panels (History, Bookmarks, etc.):
The "contextual tasks" side panel (where Google will test Chrome's agentic capabilities) now extends to the top of the browser window. Chromium developers call it the "toolbar height side panel." It currently opens on the left side, but this is temporary: