Medium has stopped offering new custom domains for publications and blogs

On November 16, 2017, Medium posted an update saying "Medium is currently pausing offering new custom domains for publications as a feature. If you already have a custom domain on Medium, nothing will change for you for the foreseeable future, and your domain will continue to work as expected."
Read more: https://help.medium.com/hc/en-us/articles/115003286907-Custom-Domains-Terms-of-Service?source=Snapzu