Zoom’s own RTMS docs explicitly say chat isn’t available. The RTMS overview also describes RTMS as a pipeline for live audio, video, and transcript data, which matches that limitation.
Zoom staff have reiterated this on the forum too - for example, in an RTMS troubleshooting thread they stated they’re “still working to support … chat data”. If you need in-meeting chat today, use the Meetings webhook event meeting.chat_message_sent or capture chat from an in-meeting client via the Meeting SDK callback onChatMsgNotification.
You could also consider use a third-party meeting bot API like Recall.ai to capture chat messages (along with video, audio, and other meeting metadata) in real-time or after the meeting has ended. We’ve helped thousands of developers with their Zoom implementations and can help you with yours too