Explanation
Two reasons that would prevent a user from being deactivated are that the user is part of a territory hierarchy or that the user is assigned in workflow email alert. Aterritory hierarchy is a structure that defines how territories are related to each other in Salesforce; if a user is part of a territory hierarchy, they cannot be deactivated until they are removed from all territories. A workflow email alert is an action that sends an email to one or more recipients when a workflow rule is triggered; if a user is assigned in workflow email alert, they cannot be deactivated until they are removed from all email alerts. The user being in a custom hierarchy field or being the highest role in role hierarchy are not reasons that would prevent deactivation; they may affect data visibility or record ownership after deactivation, but they do not block deactivation itself.
References:https://help.salesforce.com/s/articleView?id=sf.users_deactivate_considerations.htm&type=5