Whoever came up with this is wrong!
Disabling guest is sabotage! This is catastrophe!
What is so bad allowing user to decide how to use a system?
I hope truenas team change their mind about this devastating idea.
And while they do, here is my workaround.
Download config through System - Advanced settings - Manage configuration.
Open it in sqlite DB Browser and find “sharing_cifs_share” table.
Open it and edit share purpose from “DEFAULT_SHARE” to “LEGACY_SHARE”
Save it and upload back to truenas. After that, you can enable guest in shares.