What I especially miss are the blockquotes (with > some-text-to-be-quoted and code blocks (with triple-backticks, ideally in the gitlab way of allowing settings the language for syntax highlighting)
See GitLab Flavored Markdown (GLFM) | GitLab and GitLab Flavored Markdown (GLFM) | GitLab
As others have said it looks like what was added was Markdown-like shortcuts to convert to the existing Asana RTF (Rich text). I understand it’s a backwards-compatibility headache to allow just storing Markdown instead and rendering that, I don’t begrudge you having to solve that.