How to deregister a number you no longer have access to

From the document:

Deregister iMessage on your iPhone or online - Apple Support

Deregister iMessage online

If you don't have access to your iPhone, you can deregister iMessage online.

After you complete deregistration, your phone number is removed from iMessage services. You should be able to receive text messages right away but it might take a few hours for some Apple devices to recognize that you're not using iMessage when they send you a message.

and the document: Add or remove your phone number in Messages or FaceTime - Apple Support

"If the phone number that you want to remove belongs to an iPhone that you can't access, change your Apple ID password. This removes all phone numbers from your iPad, iPod touch, and Mac. Calls or messages for the removed numbers won't appear on those devices anymore."