This article explains which data types ProBackup can restore for Slack and any known limitations.
Messages
When you restore a message, it is created as a new message at the bottom of the channel. The following data is included: message content with formatting, attachments, and threads (including all thread replies).
Note: Emoji reactions on messages will not be restored.
Channels
When you restore a channel, a duplicate is created in the related workspace. All messages, files, and threads are restored with it.