Can you track a phone # that's no longer in service or in use? like I got rid of my old phone number can people still track an invalid phone #

Depends on the phone company, and if it's a landline or cell phone.

Landline numbers eventually get reused.  But years ago, I was able to track down the previous owner of my phone number - in order to tell a creditor that it wasn't MY number at the time of the debt.