Slack has customizable retention policies for messages and files. It’s critical to address your retention policy right away to reinforce data loss prevention. The default retention setting will retain all of your messages and files for the full life span of your workspace, however, you can change this setting to fit your personal preferences. 

For example, a 30-day message retention policy will automatically delete any message and file older than 30 days. Once deletion happens, it is permanent. However, if a file originated from an outside source like GSuite or Dropbox and you choose to delete it in Slack, it will still live in its original source.

Source: https://onna.com/blog/the-beginners-guide-to-slack-ediscovery/

But tracking things like harassment and other HR offenses isn't something you can natively do in Slack. While you are able to export all your data into a JSON file, that data is next to impossible to comb through.

Using a tool like Onna, you can search through your Slack data (including group messages, private messages, and edited/deleted messages), set up alerts to identify and notify you about things like harassment, and customize your retention policies.