After Updating Mac to 10.13.5 Guest User Account Appeared?
The Guest User must be enabled [&amp; will do so automatically] if Find My Mac is enabled.

From the Users &amp; Groups pref pane Help file


  Guest access works with the Find My Mac feature of iCloud to help you find your Mac if you lose it. You can locate your Mac if someone finds it, logs in as a guest, and then uses Safari to access the Internet. See Use Find My Mac.