What is my Zwift ID? – updated
The log file holds the truth – or at least it contains your Zwift user ID hidden among the many log lines.
The log file format changes sometimes so the app at https://zwifthacks.com/app/id/ has been updated (now at version 1.1.0) work with the most recent format.
The app helps finding your Zwift user ID in log.txt, directly a browser. You can drag a log file into the dropzone and the result will be shown (look for log.txt in your Documents/Zwift/Logs folder or send it to yourself from the iOS version of Zwift).