This check prevents a modern Java idiom (and likely future idioms).
E.g.

```
public sealed interface Choice
{
    record GoodChoice(...)
        implements Choice
    {
    }

    record BadChoice(...)
        implements Choice
    {
    }
}
```

The previous purpose of `InterfaceIsType` is arguably out of date
anyway. It's better to use an interface for utility containers, etc.