I use Org but markdown-mode can hide subsections too:

  markdown-mode supports outline-minor-mode as well as org-mode-style visibility cycling for atx- or hash-style headings. There are two types of visibility cycling: Pressing S-TAB cycles globally between the table of contents view (headings only), outline view (top-level headings only), and the full document view. Pressing TAB while the point is at a heading will cycle through levels of visibility for the subtree: completely folded, visible children, and fully visible. 

(Quote from the link shared by parent.)

oh, my apologies. I both use this mode and searched for the above functionality on the above link "to be sure" before commenting, but I guess I missed it.

In my feeble defense, it's hard to confirm a feature is missing T_T;;

I'm actually really glad you wrote that comment because I've been writing my blog in markdown for years and I had no idea you could collapse headings

For most of my writing (journal entries, notes), I have never needed the collapsible subheading and export functionality because I write primarily for myself. If I think a note is ready to be published on my blog, I will still rewrite, edit, and polish it in a separate file. Mostly also in Markdown, because the structure of such articles does not go very deep (I am not a fan of many levels of headings) and my static site generator handles the export to HTML in a fully automated way. No need to export anything manually.