the payload format looks like it's changed to the JSON SIM (Connect 2.0) format. This is the recommended format for Connect integrations, because it supports many more types of events (not just envelope events), lets you choose to not ingest envelope summaries for every event, and enables you to receive an event notification status at the time it occurs.
Changing the message format is opt-in, but you can change back if you want by creating a new config in the UI or by updating your connect config using the API.
Docusign has a few sample apps that I believe show how to use and implement Connect with JSON SIM format:
Webhooks sample app, https://webhooks.sampleapps.docusign.com/. The source (Ruby) is here: https://github.com/docusign/sample-app-webhooks-ruby
Business sample app, https://business.sampleapps.docusign.com/. The source (C#) is here: https://github.com/docusign/sample-app-business-csharp/