@Deprecated(forRemoval = true)
  default @NotNull Key<T> getFileDataKey() {
    // Existing plugins always override this method, so default body is never executed in the existing plugins.
    // Default implementation of `getFileDataKey` is only invoked from default implementation of `getFilePropertyKey`.
    // If new plugin observe this exception, this means that it did not override getFilePropertyKey nor getFileDataKey.
    // (or invoked getFileDataKey explicitly, either way this should be found at development phase, not in runtime in the fields)
    throw new IllegalStateException("Please override getFilePropertyKey().");
  }
@Deprecated(forRemoval = true)
  default @NotNull Key<T> getFileDataKey()