In PCBNew, when drawing a graphic line (“Draw a line” tool), for example for board outiline, it obeys the option “Graphics Editing > Limit actions to 45 degrees from start”.
However, after placing the line, if I later drag one end of the line, I have not found a way to constrain the drag to 45/90 from start. I’m not sure if that operation should obey the option I mentioned (after all what if I drag the starting point of the line?), but I would at least expect some combination of modifier keys to constrain the drag.
You could try “Shift+Space” hotkey after start of the dragging-operation.
This constraints the movement in a way that the whole line-segment remains 0°/45°/90°. For dragging-action this is a temporary modifier and has to be activated at every dragging-operation.
A constraint for the dragging-point itself so that the dragging-point (==endpoint of the line) can only move up/down or left/right is currently not available.