What is the keybind for it I can't find it in the controls menu

up
Also - is there a way to turn this off by default for new games? Could you please add the corresponding per player setting?

UP doesn't work, and there is no other way in switching this off beside making the belt marker alpha 0

I am not sure, what you are talking about. Belt highlighting can be switched off in the shortcuts.

will have a look again,could not find any shortcut related to this. but will look again

There used to be a hotkey. Not sure why it got removed. There's a shortcut key you have to place on your shortcuts bar and then use that button to toggle it displaying or not.