De register a phone number if I no longer have the phone or number

Deregister iMessage online

If you don't have access to your iPhone, you can Deregister and Turn Off iMessage - Apple Support

If you no longer have the number there is no need to deregister it, as it can’t receive iMessages or SMS texts. Deregistering is so it can’t be sent iMessages, and anything sent will go to the number as an SMS. But if the number doesn’t exist it can’t receive them anyway.