How do I unpair my Apple Watch from an old iPhone I can't get into?

How to get into the iPhone with a forgotten passcode:

…If you forgot your iPhone passcode - Apple Support

How to unpair with or without the paired iPhone:

…Unpair and erase your Apple Watch - Apple Support

From a web browser,sign in to iCloud.com with your Apple ID.

Go to Find iPhone.

Click All Devices at the top of the screen.

Click your Apple Watch.

Click Erase [device]. Select Next until the device is erased.

Click Remove from Account.