There's already an existing related ticket for this here - Make `message.text` not required for Slack action, but responses are closed and I wanted to get an idea of when or if a fix for this is on the horizon. Right now I can't access the Watcher UI on v6.2.4 because of an error: "Watcher: Error 400 Bad Request: json argument must contain an actionJson.slack.message.text property". My watches have a message.attachments.text, so the additional message.text shouldn't be necessary, and that error shouldn't prevent me from accessing the UI to update the Watcher.
I've attempted updating the watcher via the DevTools, but I have a lot of watches to update and moving around all of the JSON is a bit tedious. I've also tried using _update_by_query, but that doesn't work either because I'm using the Cloud product so I'm unable to set "watcher.index.rest.direct_access: true" in my elasticsearch.yml.