Config form saves wrong value types

Should indeed be fixed in the implementation and schema. In general a config schema check should be done.

Eventually the existing values have to be updated? If not, even better and less work!

This is not true. All boolean fields are saved as boolean, and there are no schema errors, see exported "purge_users.settings.yml":

user_never_lastlogin_value: ''
user_never_lastlogin_period: days
user_lastlogin_value: '12'
user_lastlogin_period: days
user_inactive_value: ''
user_inactive_period: days
user_blocked_value: ''
user_blocked_period: days
enabled_never_loggedin_users: false
enabled_loggedin_users: true
enabled_inactive_users: false
enabled_blocked_users: false
purge_included_users_roles:
  authenticated: authenticated
  content_editor: content_editor
  administrator: '0'
purge_on_cron: false
inactive_user_notify_subject: 'Your account is deleted'
inactive_user_notify_text: "Dear User, \r\n\r\nYour account has been deleted due the website’s policy to automatically remove users who match certain criteria. If you have concerns regarding the deletion, please talk to the administrator of the website. \r\n\r\nThank you"
send_email_notification: false
purge_user_cancel_method: user_cancel_reassign
disregard_blocked_users: false
purge_excluded_users_roles:
  administrator: administrator
  content_editor: '0'
user_before_deletion_subject: 'Your account will be deleted'
user_before_deletion_text: "Dear User, \r\n\r\nYour account will be deleted soon due the website’s policy to automatically remove users who match certain criteria. If you have concerns regarding the deletion, please talk to the administrator of the website.  \r\n\r\nThank you"
send_email_user_before_notification: false
user_before_notification_value: ''
user_before_notification_period: days

NOTE, that the '0' values are coming from the checkboxes values, which saves values as '0', when they are not selected.