Motivation based on the internal discussion:

When doing lazy asynchronous imports, if you try to use some iconset before it was loaded/defined, it won't update iron-icon after the iconset is ready. We have a special bundle for icons on the website, with the import of iron-icons as the last one. But I cannot add vaadin-icons there for the reason above

We can still keep iron-icon as a dependency for backwards compatibility, and even have a separate file importing it as well, in order to avoid breaking change.