DECRQM:
Reports the state of ANSI and DEC private modes back to the application (modes from SM/RM and DECSET/DECRST).

For more advanced termlibs it is crucial to be able to request feature state from the terminal. xterm.js currently lacks support for DECRQM.
This is more and more important for app side, as more features are added to the modes list. Without DECRQM applications have no means to determine at runtime, if a certain feature is supported by the underlying terminal.