In my college comp-sci networking class, one of our projects was to figure out a system for sending messages visually between two parties spaced 100 yards (91 meters) apart as quickly as possible.

Each team of four came up with their own signaling method, trying to balance how many bits were transmitted with each symbol with how quickly we could encode/decode the message.

Binoculars were not allowed. We were outside, so we didn't know how windy it would be.

I seem to recall on my team, we were sending two bits per symbol. I think it was just a large cardboard box, we colored each of the four flaps visually distinct, then unfolded one of the flaps for each symbol.

I don't recall a single team thinking to re-purpose naval flags.

Duh.

https://en.wikipedia.org/wiki/Flag_semaphore would have been good if you needed to send arbitrary text.

Naval flags would be higher bandwidth if you could pre-arrange a modest-sized dictionary of the words and phrases you’d need.

https://www.navalgazing.net/Signalling-Part-1 is a good read.

It’s been over two decades but I think the requirement may have been an arbitrary bitstream but if not that, it was likely any of the visible ASCII characters.

There is also flag semaphore which is very fast and very recognizable and gives you about 5 bits per signal.

Morse may not be the oldest but my bet is on it being by far the most widely still used of anything nearly two centuries old. I got into ham radio a decade ago and learned Morse (poorly) just because. It already wasn't a requirement, but so many still use it.

Hopefully there's an app for listening to it and interpreting it? I bet those old timers get pretty fast with the Morse

Braille is originally from a similar timeframe and carries six bits. Modern enhancements have focused on the protocol above the payload to make it more efficient.

If that counts, then so do Chinese characters (1200 BC or older)