Xcode Compile Flag to Suppress Nullability Warnings not Working?
To disable those warnings, you want: -Wno-nullability-completeness. Note the no-; the flag you're using enables those warnings.
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_BEGINNS_ASSUME_NONNULL_END
Settings for Xcode 11.5 when using project with mixed sources, ObjC and Swift:
WARNING_CFLAGS = $(inherited) -Wno-nullability-completenessOTHER_SWIFT_FLAGS = $(inherited) -Xcc -Wno-nullability-completeness