Why is there no “hide my caller ID” on my iPhone XS with iOS 12.4

Because the ability to hide your Caller ID is determined by the carrier. If the switch does not exist on your phone, it is because the carrier does not allow it to be switched that way. You can contact your carrier to see if they allow for turning off Caller ID. The actual heading of the setting is Show My Caller ID, and that is in Settings>Phone, and scroll down the Calls section. If it does not exist, contact your carrier to see what they support regarding Caller ID.

Contact your carrier and ask them. They do provide you with that feature.