Venmo's public API is available for you to look at, but most people probably don't understand what information is public just by looking at that blob of text. I wanted to put it into a format that could properly convey what information you're sharing by using Venmo.
When using Venmo to make payments, you are broadcasting your name and picture to anyone who wants to see it. Someone can also work out what you are paying for by looking at the message, which is also publicly available. In the hands of someone with malicious intent, this is dangerous information.
If you are using Venmo, check out Public By Default to learn more about the Venmo public API and how to make your transactions private.