A missing annotation type at runtime does not cause problems when loading the annotated type, it is just not present in this case.

That would be nice. I guess we'll soon find out.

As we'd continue to access multi-valued annotations via the corresponding @List type, I'd expect it actually to work.

Sure. However, at some stage the code would need to actually try reading the @Reatable annotation to determine the containing annotation type.

@Repeatable is just a hint for the compiler to create an instance of the specified list annotation, so for code using the legacy APIs, there is no difference whether the annotation itself is given several times directly or via the @List annotation.