"It would appear that the latest version of the Messages, version 18.104.22.168, has removed the 'Schedule Message' option."
Taken from this thread:
I use the schedule message a lot, and dont want to lose it. Can I prevent my S9 from updating?
Yes, it is easily possible to stop your device from getting the update of the latest version of messages. You can go to the play store and find the "my apps" to find the message application installed on your device. after getting the application, you can simply disable the auto-update option, to have a control on updating it whenever you want to. You may also consult with Gmail Support Canada who may help you with enough of the information.
I am on version 22.214.171.124 which it says is the latest the schedule message is still there but you have to click on the + sign next to the box where you type text then you get the three little dots on a bar push that and it’s in there