Zulip supports ~CommonMark Markdown: https://zulip.readthedocs.io/en/latest/subsystems/markdown.h...
Reddit supports Markdown. https://www.reddit.com/wiki/markdown
Discourse now supports CommonMark Markdown.
GitHub, BitBucket, GitLab and Gogs/Gitea support Markdown.
You can register for a free personal account with only an email, no requirement for phone number or contacts upload.
Related discussion: I've built FastComments to be "realtime" and support markdown. Been thinking of reusing the infrastructure to launch a little chat app, but not sure what the killer feature would be aside from being lightweight...
Edited to add Volt link.
[1]: https://faq.whatsapp.com/general/chats/how-to-format-your-me...