There’s also this thread which has slightly more recent discussion about markdown, from one year ago. So I guess it’s still being implemented.
I think it’s being developed by @adbenitez who you’ll see is also active on here and also contributes to Delta Chat. Everyone is free to make their own client, and I guess Arcane Chat gives Asiel more flexibility to move quicker and implement features that would take longer to be implemented by the official Delta Chat client, and it can also be a nice incubator of new ideas. It would definitely be great to see features in Arcane Chat such as markdown support or the ability to selectively enable/disable specific accounts upstreamed to Delta Chat!