Some compiler was warning about this being unused https://github.com/mongodb/mongo/blob/29a838f9193ba88c7ba8195fd44eb252a80be5f8/src/mongo/util/system_tick_source.cpp#L75

If I had to guess, I'd say it wasn't removed when it was replaced with a local static https://github.com/mongodb/mongo/blob/29a838f9193ba88c7ba8195fd44eb252a80be5f8/src/mongo/util/system_tick_source.cpp#L156