From the "What's new for developers" in macOS 10.13 High Sierra document [0], case-sensitive APFS can be normalisation-insensitive:
> APFS now supports an on-disk format change to allow for normalization-insensitive Case Sensitive volumes. This means that file names in either Unicode NFC or NFD will point to the same files.
Which means that both versions support normalisation-insensitivity.
(Edit: There is also a one-line mention of this in the iOS 11 document [1] but it doesn't say if it is the default.)
> APFS now supports an on-disk format change to allow for normalization-insensitive Case Sensitive volumes. This means that file names in either Unicode NFC or NFD will point to the same files.
Which means that both versions support normalisation-insensitivity.
(Edit: There is also a one-line mention of this in the iOS 11 document [1] but it doesn't say if it is the default.)
[0]: https://developer.apple.com/library/content/releasenotes/Mac...
[1]: https://developer.apple.com/library/content/releasenotes/Mac...