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Why `abstract` can be `inner`, but `sealed`, `data` not?

<https://discuss.kotlinlang.org/t/is-there-a-reason-for-not-allowing-inner-data-classes/2526|https://discuss.kotlinlang.org/t/is-there-a-reason-for-not-allowing-inner-data-classes/2526>

In general it's just a design decision, rather than a technical limitation

For data class I can compromise, but sealed class I don't think it's convincing. My use case suggests me to use exhaustive when while each subclass holding a reference of the outer class because I need the generic type. For now I need to re-declare that generic type, and you can see it's treated differently on this aspect between abstract and sealed.

For sealed it's by historical reasons, originally it was possible to declare a sealed class only on top level
<https://discuss.kotlinlang.org/t/sealed-inner-classes/2371/7|https://discuss.kotlinlang.org/t/sealed-inner-classes/2371/7>