Error 'You cannot deactivate a User who is receiving cases or notifications as part of your Case Assignment or Escalation rules'

After checking the Case Assignment Rule and Escalation Rules.

User is listed on an Inactive Assignment Rule or Escalation Rule, and cannot be deactivated.

Most likely, this is due to the Escalation Action inside the Escalation rule. Comparing the Assignment Rule and the Escalation Rule, in the Assignment Rule, you can see right away the "Assign To" for each rule. However, for the Escalation Rule the "Assign To" is not readily visible after accessing the escalation rule. Click "Edit" for each Rule Entry. Once you are on the edit page for the rule, you will now see in the "Escalation Action" section if the user is added to either the "Assign To" or "Notify" column.

After the corresponding action then replace the user on the "Auto-reassign cases to" and/or "Notify This User" section. Click Save. Once done, deactivate the user. 

Another option would be to use a Metadata API Tool, like Salesforce Extensions for Visual Studio Code and try to find references to the User that you're trying to deactivate.

See also:
Error when you deactivate a user who is the default case owner 
Escalation Actions   
View and Edit Assignment Rules