See https://tools.ietf.org/html/rfc4180, Common Format and MIME Type for Comma-Separated Values (CSV) Files.
The first line in the CSV file must contain the column header named id. The subsequent lines must have the SCIM ID of the users to delete.

For example:
id
60P7273T7P
60A7471A9Q

By default, an email is sent to regular users that the account was deleted. Federated users do not get an email notification. The email template for branding is at "notifications/user_management/profile/{locale}/account_deleted_email.xml". Pass in the themeId query parameter to brand the email template for notifications.

To turn off email notifications, send notifyType=NONE as a query parameter.
The maximum file size is 10MB.

Entitlement required: manageUserGroups (Manage users and groups) or manageAllUserGroups (Synchronize users and groups) or manageUserStandardGroups (Manage users and standard groups), or manageUsers (Manage all users), or manageUsersInStandardGroups (Manage users in standard groups).
Note: You only need one entitlement, but you can have more than one.