Awesome plugin which we've been using for a while now. Ryan Heller helped us for our website iCulture.nl to add (missing) custom post types support to WP Slack. Ryan kindly made this available via a pull request on Github, but the developer hasn't made this publicly available via the plugin on the WordPres.org repository yet. But for whomever needs it, please find the pull request here: https://github.com/gedex/wp-slack/pull/24