UPDATE (August 2020): Custom Domains will be available again for Medium publications soon. We don't have an official date yet, but we'll make an announcement when we re-launch this feature. Stay tuned.
As of November 2017, Medium is no longer offering new custom domains as a feature. Instead, you can create a publication on Medium that will live on a medium.com/publication-name URL.
Existing publications on custom domains
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.
However, if you currently have a custom domain on Medium and you maintain CAA Records, you must add one for comodoca.com or we won't be able to renew your SSL certificate every year. If you don’t have any CAA Records there’s no need to add anything. Your certificate will renew seamlessly.