This is a simple exporter backend for Emacs org-mode, derived from the builtin markdown exporter & its derived backend ox-gfm.el. It produces simple text output suitable for pasting into a Slack client. It works like most export backends, with two main differences:

• at present, this exporter is not added to the export dispatcher. It’s hard to see when it would be useful to make slack-formatted files. Instead,
• this package provides the interactive function org-slack-export-to-clipboard-as-slack, which copies the region to the kill-ring in slack format. From there it should be easy to paste the resultant text into your slack client.
• on the slack side, you can quickly apply the formatting by pressing Ctrl Shift F (Linux/Windows) or ⌘ Shift F (Mac).

Since slack syntax is much poorer than Org’s, I’ve made some arbitrary choices about how to degrade it. Headlines are exported as bolded text on their own lines, for instance.

There are probably lots of improvements possible. Please file issues in the Github repo or discuss in the org-mode mailing list.

Now available in MELPA! Install with package-install or just (use-package ox-slack)