Wire supports a subset of markdown syntax, <a href="https://support.wire.com/hc/en-us/articles/115002676029-Use-markdown-to-format-text" rel="nofollow">https://support.wire.com/hc/en-us/articles/115002676029-Use-...</a><p>You can register for a free personal account with only an email, no requirement for phone number or contacts upload.
To respond to OP:
<a href="https://volt-app.com/" rel="nofollow">https://volt-app.com/</a><p>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...<p>Edited to add Volt link.
WhatsApp supports italic, bold, strikethrough, and monospace using backticks you could use for source code, though no syntax highlighting[1].<p>[1]: <a href="https://faq.whatsapp.com/general/chats/how-to-format-your-messages/" rel="nofollow">https://faq.whatsapp.com/general/chats/how-to-format-your-me...</a>
WhatsApp isn't "markdown", but supports code blocks using triple backquote (and uses underscore for italics and star for bold, which I think is a lot better than markdown anyway).