Comment by haberman

Since preserve_none is Clang-only and only available on recent compilers, it would introduce an ABI hazard between the core library and generated code. We don't want to cause crashes if you compile protobuf with one compiler and generated code with another.

Also, until quite recently preserve_none was incompatible with the sanitizers, but I believe this may have been fixed by: https://github.com/llvm/llvm-project/pull/81048