Xcode Compile Flag to Suppress Nullability Warnings not Working?

Note that in Xcode 11 it is now -Wno-nonnull

It should be set in "Other Warning Flags" in the "Apple Clang - Custom Compiler Flags" section of your target.

You could also avoid these warnings and setting the above altogether by removing these flags Xcode automatically adds to new header files in Objective-C:

NS_ASSUME_NONNULL_BEGIN
NS_ASSUME_NONNULL_END

To disable those warnings, you want: -Wno-nullability-completeness. Note the no-; the flag you're using enables those warnings.