Computer Configuration > Administrative Templates > Microsoft Edge
Both
HKLM\Software\Policies\Microsoft\Edge
Standalone Sidebar is an optional mode for the Sidebar in Microsoft Edge. When this mode is activated by a user, the Sidebar appears in a fixed position on the Microsoft Windows desktop, and is hidden from the browser application frame. If you enable or don't configure this policy, users will have the ability to activate the Standalone Sidebar. If you disable this policy, options to activate Standalone Sidebar will be hidden or made unavailable. Note that blocking 'HubsSidebarEnabled' (Show Hubs Sidebar) will also prevent users from accessing Standalone Sidebar.