How can I deregister a phone number from an iPhone that got stolen

I'm very sorry about your iPhone XS. Have you taken such steps to lock and erase this device using the Find my iPhone feature?

Also you should go ahead and Deregister iMessage from it here. Other than these options and a possible SIM card, there should be no other connection needed to break between the iPhone and your iCloud involving your phone number. If you don't get it back and decide to get another iPhone, you should be able to more your cellular service (number included) to the new device as well as your Apple ID.

Here is some more information regarding managing your devices

Is Two-factor Authentication turned on for your iCloud account? Perhaps you may need to add your new number into your secured account and delete the old one.

"If your iPhone is your only trusted device and it is missing or damaged, you will be unable to receive verification codes required to access your account.

You can update your trusted phone numbers when you follow these steps:

1. Go to your Apple ID account page.
2. Sign in with your Apple ID.
3. Go to the Security section and click Edit.

If you want to add a phone number, click Add a Trusted Phone Number and enter the phone number. Choose to verify the number with a text message or automated phone call, and click Continue. To remove a trusted phone number, click  next to the phone number you want to remove."