On top of that I've had friends switch away from iPhone to Android; iOS switched to using SMS not but a day later (and I know at least some of them didn't turn it off manually because their iPhone was broken).
If you know it needs to be done. If you still have the device.
According to our head of IT, he said if you didn't (as most people don't) it gets a whole lot worse.
Better yet, unregister your device through https://supportprofile.apple.com/MySupportProfile.do
Sarcasm aside: There are comments unlike like yours, which takes it as a granted that everyone should know about these things and how Apple does everything internally.
Ironically enough, they perfectly highlight the self-centeredness of the Apple community and ecosystem. Which was the one thing I complaining about in the first place.
Turn the old phone off and problem solved.
iMessage specifically confirms delivery. That's why you occasionally get the "Send as SMS" prompt if it can't go through for whatever reason.
If there's no device actively logged into iMessage, no attempt is made to send through iMessage. You don't have to "deactivate" anything.
That said, when iOS7 came out, and I logged back into my iPad, and my wife on her iPad Mini, and her iPhone 5, iMessage reenabled on all of them with my phone number.
So that was a bit inconvenient.
That obviously wouldn't happen if you had reset and sold the device.
If you sold the device without a reset you have a lot more to worry about than iMessage I think. Though a password change to your Apple ID, or managing the associated phone numbers there should address the issue.
BTW, SMS on Android... wow. That's probably the shittiest thing about switching. It's hard to appreciate how unreliable, low quality and all around bad SMS/MMS is if you've been using iMessage for a few years.
Messages get split up over 140 characters. You can't forward vCards. MMS take forever and you're lucky if you even get half the messages in a timely fashion in-sequence. It's really truly awful.
So... I could go get a GoPhone plan with a new number. Transfer my existing number to Google Voice. When the transfer is complete my old plan will be cancelled, I can swap in the GoPhone SIM, add it's number to Google Voice as a forwarding number, and I've got it all done with improved text/group-messaging? Does that sound about right?
Really was one of the most frustrating experiences I've had with customer service.