Same issue here. My PC (Win 11 Home) doesn't even have Windows Copilot as an option, so there's no setting in Personalization > Taskbar to remove it. This is just a button (which does nothing but open Edge, a browser I don't use), which StartAllBack itself is adding to my taskbar for unclear reasons.
Unfortunately for me, right clicking is no different from right clicking anywhere else on the taskbar; there's no option to remove the icon.
EDIT 1: I tried two different registry changes from around the Internet and they did nothing. Presumably because they're designed for when the OS is inserting the Windows Copilot preview button, but it seems like in this case StartAllBack is inserting the button.
EDIT 2: It was interference with ExplorerPatcher. Was able to turn off the button with right click after uninstalling.