After upgrading to jekyll-3.5.0 with liquid-4.0.0 customData of the menus (defined in front matter) wasn't accessible any more because the commit of https://github.com/Shopify/liquid/pull/661 renamed the Drop method before_method to liquid_method_missing. So I did the same renaming in my local jekyll-menus-0.5.0/lib/jekyll/menus/drops/item.rb and it worked again.
After upgrading to jekyll-3.5.0 with liquid-4.0.0 customData of the menus (defined in front matter) wasn't accessible any more because the commit of https://github.com/Shopify/liquid/pull/661 renamed the Drop method before_method to liquid_method_missing. So I did the same renaming in my local jekyll-menus-0.5.0/lib/jekyll/menus/drops/item.rb and it worked again.