Remove titlebar without overrideredirect() using Tkinter?

You must give your root window name before your command.

Like this:

from tkinter import *

root=Tk()
root.wm_attributes('-fullscreen','true')
root.mainloop()

The window decoration is all handled by the window manager so what you are trying to do is find a way to tell the window manager to decorate your window differently from a standard application window. Tk provides overrideredirect to have the window manager completely ignore this window but we can also use Extended Window Manager Hints to declare the intended use of this toplevel window to the window manager. This is done for instance for tooltip and splashscreen windows to allow the manager to provide minimal decoration and possibly special animations.

In your case, adding a 'splash' hint should do what you want

root = tk.Tk()
root.wm_attributes('-type', 'splash')

You will need Tk 8.5 or above for this.