To set up personal Slack notifications in WebSked the Arc user must have a valid profile in Authors service. However, to set up a valid profile the user must have a valid email address.
We have multiple use cases where our authors do not have email addresses. As such, WebSked prevents them from setting up personal Slack notifications. It make sense to require an email address to use Slack.
Thanks for the feedback. We currently leverage the email address associated with the user account to match up edits across the Arc XP experience, but we'll take this is under consideration for future updates.