https://selfsolve.apple.com/deregister-imessage/
Should be Job Done.
What Apple’s doing should be fucking prosecuted by the FCC or FTC. It’s outrageous.
The last thing it should do is silently send a message to an inactive account such that it looks like I ghosted the person.
Return an error or redirect to SMS, it's not that fucking hard.
The one thing you are right about is that it's an intentional dark pattern on Apple's part to obsfucate Apple IDs and phone numbers. It's an illegal anticompetitve strategy.