I'm just curious if you're having issues with actually sending/receiving because of iMessage not deregistering, or more because you can no longer communicate with iPods/iPads.
Yeah, I don't remember how I came across this, but I'm thinking it was through Samsung Smart Switch. They direct you to a link that makes deregistering iMessage really easy, in case you didn't do it through the iPhone first. I know you've already resolved it for yourself, but in case anybody else is having a problem this link makes it super easy.
https://selfsolve.apple.com/deregister-imessage
Enter your phone number, enter confirmation code that they send you on that page.
Anyway, for me personally I can live without iMessage. I'm not sure I understand why anybody would miss it unless they need it to contact people with iPods/iPads (which I do understand may be a big deal).