_MOTIF_WM_HINTS
An X11 property from Motif that lets a program ask the window manager to drop the frame — and that outlived Motif by decades.
Motif's window manager, mwm, read the _MOTIF_WM_HINTS property on every top-level window: a small struct with flags, functions, decorations and input_mode fields. Through it a program could say it wanted no title bar, no border or no maximise button, or that it must not be closed. ICCCM had nothing equivalent, since frames were considered the window manager's business.
When EWMH was drafted around 2000 there was still no standard way to request an undecorated window. So GTK, Qt, SDL and game engines kept setting Motif's property, and almost every X11 window manager — Motif lineage or not — still reads it today. It is quite possibly the most widely used piece of Motif there is.