@eliot-slevin I see a variety of delimiters used in practice. > is popular, as is :, ^ and <. Many EDI platforms use : as the default.
The important rule to remember is that the delimiters cannot show up in your textual data. If you know that your text data uses mathematical symbols then > and < is probably not a good choice, and : or \ would be better. But, if you know your text data includes time values in the format HH:MM:SS you may want to avoid :.
Communicate with the business groups owning the data being sent. Clearly communicate the character restrictions and get company buy-in, and agree on substitution characters when a delimiter is found in textual data.
Finally, if you are receiving EDI, be very open to the delimiters you allow. The sending party will understand the data being sent and the restrictions, and they have likely done upfront work to ensure there are no conflicts.