I don't think the taskbar height information is accessible in Godot, since different operating systems would have different taskbars. Additionally, some operating systems allow for different sizes of taskbar as well, further complicating the issue. There may be a system variable for this, but I am not sure.
All of that said, on both Ubuntu Linux and Windows 10 I haven't had this issue, I can maximize the window and it correctly takes the size of the taskbar into account. I think the maximizing feature is OS specific, rather than application specific, so maybe its something on the OS side that is causing the issue? Do other programs have this issue or is it only Godot?