What's the oldest protocol that's still regularly used?
https://en.wikipedia.org/wiki/Baudot_code#ITA2
> In 1924, the CCITT introduced the International Telegraph Alphabet No. 2 (ITA2) code[14] as an international standard, which was based on the Western Union code with some minor changes.
> ITA2 is still used in telecommunications devices for the deaf (TDD), Telex, and some amateur radio applications, such as radioteletype ("RTTY").
It's interesting that the fundamentals of this were invented in 1870s. Who needs semiconductors when you have gears and levers?