I brought this up with a chat agent on the support pages. I pointed out that the iPhone User Guide, describing the Send Later feature, says "Your recipient can use any device, and they won’t know that the message was scheduled." (Schedule a message on iPhone to send later - Apple Support )
After the usual BS about upgrading from 18.0.1 to 18.1, etc failed to change the behavior, the agent finally said:
"I have checked with my resources and I would like to inform you that, the iMessage is a feature specifically built for only apple devices. And the iMessage send later feature is only available for iOS devices. ("Your recipient can use any device, and they won’t know that the message was scheduled") this means only apple devices like iPhone, iPad, Mac. If the other user/recipient is an android user, they won't be able to use the feature from your end also."
So, yeah, it's yet another example of Apple giving the middle finger to the standards-based, interoperable rest of the world. "You're either with us or against us."
Evil.