Untraceable for what?
Almost all of them still use metal parts that can be x-rayed and still have barrels that leave ballistic fingerprints on bullets. Serial numbers don't make something GPS-tracked.
Untraceable in terms of ownership? There is no national firearm registry. Guns bought from FFLs require a NICS background check that is stored in an ATF database (of questionable legality), but private sale guns often don't require NICS so the database isn't an accurate registry of gun ownership.
And criminals scratch off serial numbers anyways.
And add on that any laws requiring serialization of privately-made firearms are only affecting nerds, not criminals. Criminals that are making guns because they can't pass a NICS background check will continue not adding serial numbers - because they're criminals.
Yep, if you're a criminal making ghost guns to commit crimes because you can't pass a background check to buy a gun (and then scratch the serial number off), then it's already a crime for you to have that ghost gun because you're a prohibited person.
Requiring a serial number changes nothing and only affects nerds, not criminals.