Even when I follow your suggestion, it's not there.
Speaking as a Note 9 running One UI on AT&T. I can't say what any other phone or carrier has.
Compose your message, click the + sign on the left side of the message.
3 dots will appear on the right side of the message bar. Click that. An assortment of options will appear, including "Schedule message."
Fill in the date and time and click SEND.
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