You can’t actually delete a user in Slack. You can, however, disable a user. And the cool thing about disabled users is that you can still access the messages you’ve sent to them.
Accessing the messages is simple. First go to your Team Directory page in Slack. The url for that is something like http://yourteam.slack.com/team. Now find the greyed out “Disabled Accounts” section. Click on it. Next to each of the users you will see a […] menu. Open that menu and select “Open Message Archives”.
You can now view your archived direct messages from previous members of your team.
It is also possible to access messages from a disabled user using the Slack search functionality. In order to do this you can simply enter
usernameis the name of the user you are looking for) in the Slack search bar. The search results should actually show you messages from the disabled user.