Every other tool has it. Gimp has it. Blender’s Camera-mode has it. Krita “probably” has it.
How about Synfig?
We could just “Switch ON” from the top panel, where other buttons (such as Show Grid, Render Settings, etc.) are available. Perhaps a drop-down menu?
The menu itself could contain a few grid options: Rule of Thirds, Rule of Fifths, Golden Mean, Fibonacci Spiral, etc. This can be useful for those who love composition and take great care to get things “just right”.