1) These buttons are broken with default GTK theme. As nobody has
reported bug I assume these buttons are unused. GTK is default
theme type since 3.2.0.

2) These buttons are not supported by client side decorated
windows.

3) Few Metacity themes I checked had no icons for these buttons.
They had empty draw_ops...

4) These buttons has been removed also from Mutter.

Functionality itself is not removed! Still available using window
menu and/or keybindings.