As of MS Build 2018 custom functions in Excel are available for all developers to use and implement.
This is actually a huge deal because one of the limitations of the Add-In model in the past is this ability to create a custom function without having to add it to the tool bar as an icon which has to be clicked. With this release it means that develoipers can create their own functions in excel in an analogous manner to VBA. The simple example below adds the two numbers and for no good reason adds 42 as well. But you get the idea.