Recently we migrated a Revit addin from version 2015 and 2016 and found that the class ExternalDefinitonCreationOptions was gone in the Revit .NET API 2016. We wondered why it was removed as it looked a good addition to the API by version 2015 or a bit earlier. Searched a bit but found it was renamed as ExternalDefinitionCreationOptions. It made sense since there was a typo there.
In fact, we did not notice it before, simply because the Visual Studio intellisense and auto-complete features got it done automatically for us . Thanks to it! It's not a big deal indeed. Nobody would really blame it even if it was named something like EDCO. However, getting the spelling fixed has brought some code stream incompatibility and maintenance issues. The #if #else and #endif had to come to rescue.