window hold_mode <on|off|toggle>

This turns hold mode on or off for a window. When hold_mode is OFF, new output to a window causes the window's contents to scroll upwards to make room for the new output. When hold_mode is ON, new output to a window may be “held” and not displayed until you later use the bind send_line keybinding (the <enter> key), or the bind scroll_forward keybinding.

The window hold_slider topic talks more about unholding output.

This operation was already in existance in ircII2.1.5g.