Session tracks usage and crashes to make sure app stays performant and bug free. Microsoft App Center (https://appcenter.ms) and Mixpanel (https://www.mixpanel.com) are used to collect the data.
No Slack data is stored beside team name + token on server.
By default, Session won't send any data to server if user is not enrolled in Pro account. Otherwise, Session only store your sessions (work and rest) for backup and to sync your data between devices.