Number is no longer active so can’t have service to do this
I trying deactivate my number but no service with that number anymore so how can deactivate it
iPhone 12 Pro, iOS 16
Where is the SIM with the Old Number, If it is not with you or you have given it back to the Carrier Service Provider, Then don't bother about it? Then de-registeration is not required and also not possible.
If you have it with you and want to use it with an Android device then put the SIM on any Phone (Apple or Android) and follow the GIF
Insert the same SIM into an Android or an iPhone. You may deregister iMessage online
Watch the GIF below. Here I have used another iPhone for the demo but for you, it may your choice. Android or iPhone device.