I’ve created a bare-bones plugin if you use BuddyPress and interested in posting activity to your Slack channels. It’s available from my Github account, but also from the WordPress.org repo.
I created this plugin in less than a day and it was really for my own purposes. I don’t plan on supporting it officially although (1) I do plan on adding features as time permits in the future and (2) I’m open to being hired by another party to customize it for them, or add features.