Had that stupid Android problem recently. I will have to try that fix.
Yes, this is the biggest issue I've encountered with iMessage (other than the occasional flaky service). I support a few dozen iPhones at work & see this ALL the time. Basically when you switch to a non-iPhone cell phone, iMessage doesn't automatically turn off. You then have 3 options:
1. Turn it off yourself, in settings (assuming you still have the phone)
2. Turn it off in your
device list on Apple's website (best method imo)
3. Wait 45 days for it to clear out of the system
The only confirmation I have about the "45 days" bit is from people posting it on support forums. Apparently Apple tech support isn't aware about the "unregister" trick on their Support Profile site and pushes an iMessage deactivation message into the system for your account. I don't know if the 45 days applies to non-usage as well (i.e. it will quit working after 45 days of non-use automatically).
I have found this to be a huge hassle when friends have switched to a different phone brand & have stopped receiving my messages; Apple does not make this information readily available, even through searching. This is a glaring issue for the iPhone; they are actually being sued over it right now.