This does not occur in 4.18.3 at CL 3823254
UATHelper: Packaging (iOS): ERROR: System.IO.DirectoryNotFoundException: Could not find a part of the path 'D:\Epic Games\UE_4.19\Engine\Binaries\ThirdParty\ICU\icu4c-53_1\Mac'
This did not occur when built locally on a Mac at CL 3895718
Result: Package fails in the Build step
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-53873 in the post.