Have bath tubs ever been considered "self-venting" in plumbing code?
I don't believe that the overflow was ever considered a vent for a bathtub.

The main purpose of the vent is to prevent the water in the trap from being sucked out when water runs through the trap. Therefore the vent must be located downstream of the trap. The exact location of the vent connection is determined by code and has changed over the years. However, the connection was never accepted upstream of the trap.

There was a time when every fixture was not required to be vented (there was also a time when there was no formal plumbing code) so I can’t say never, however, I can say that since traps were required to be vented, the requirement was downstream of the trap.

I’ve looked at the other question, and can say with confidence that the bathtub would have been required to be vented either individually or via a wet vent thought the bathroom sink.