I believe I have figured out why.
The documentation where you found _cvtu32_mask16 indicates that _cvtu32_mask16 is an llvm(clang) function. And the error message indicates it was XNNPACK that is linking this function.
So I dive into the XNNPACK source code and found this line: https://github.com/google/XNNPACK/blob/master/src/xnnpack/intrinsics-polyfill.h#L36 8
When I checkout pytorch tags/v1.5.0, the same line says Apple Clang pre-10 so the judgement is (__apple_build_version__ < 10000000). In this way, XNNPACK does not define its own _cvtu32_mask16 implementation but refer it to clang, tries to link it finally and failed because the clang installed on macOS 10.13/10.14 does not have _cvtu32_mask16 implemented in its libraries.
That also explains why macOS 10.15 works because the clang version on macOS 10.15 is 11.