Turning a sweet cake into a savoury one: What should I swap in place of sugar?
On King Arthur's site, they talk about liquid sweeteners.
One of their comparisons between the different forms is water content/acidity.
Maple syrup's water content/acidity: 34%, mildly acidic (less acidic than honey).
This led me to look up water content and acidity impacts on baking.
The Cake blog did a comparison of cakes based on level of acid used. The no acid cake (what I would assume would happen if you removed the maple syrup because it contains malic acid) still worked, it just wasn't as tall or fluffy as the ones with acid.
My suggestion is that if you want a loaf that is more cake-like, you should add a bit of acid (e.g., lemon juice or vinegar) if you remove the maple syrup. If you want a loaf that's denser, you can leave out the maple syrup and don't add in anything else.