At no point are messages that are scheduled through the app or sent through Slack in any way stored outside of Slack itself.
When a message is scheduled, the app extracts the message, time to schedule and any information needed to scheduled the message. It is then sent directly to Slack via their secure API following their recommended best practice.
To enable this - the Slack Scheduler app must first be installed by a member of your team. It then needs to be authorised by each individual team member when they want to begin scheduling messages.
We know that Slack isn’t just for Giphy. Important work is done and discussed on Slack.