It would be great if Microsoft could make up their minds... custom domain available, custom domains canceled, custom domains available.
With the new program they began rolling out in September 2018, you need to buy your own domain from GoDaddy (or move it to GoDaddy registrar if you already own a domain.) As before, you'll get 5 mailboxes.
If you need more mailboxes or want more control, you can migrate to the business side, where email-only is $4/month per mailbox. This plan is closest to the Outlook.com offerings and includes the ability to configure junk mail filtering and other options, which are not available in Outlook.com. In my opinion, it’s a better (but more expensive) option for calendar and contact sharing with other family members.
Some of the “premium” features are being rolled into Office 365 Home and Personal subscriptions, including ad-free Outlook on the web (which should have been ad-free from day 1). In addition, Office 365 Home and Personal subscribers will have a 50 GB mailbox, enhanced email security and premium support.
However, subscribers need to have an Outlook.com address (including Hotmail, live, and msn.com) associated with their subscription to receive the premium upgrade. If you need the premium features and used a gmail, yahoo, or other non-outlook.com address to set it up, add an Outlook.com alias to the Microsoft account and set it as default. (It won’t mess up Microsoft account logins; at most you’ll need to sign in using the outlook.com address. I changed the addresses on several accounts last year and it was surprisingly smooth.)
It is expected to take several weeks for these new benefits to roll out to subscribers.
The adding of a custom domain seems to be back for a selected few who have Office 365 Home.
I got an option to add a custom domain (only from GoDaddy at the moment) and any of my family members that are apart of the subscription can also add an email address from that domain.
https://support.office.com/en-us/article/get-a-personalized-email-address-in-office-365-75416a58-b225-4c02-8c07-8979403b427b?ui=en-US&rs=en-US&ad=US
Yes, it is back - they started rolling it out in September. The only change is you need your own domain and it only works with GoDaddy currently (they update the DNS records automatically). They may eventually allow other registrars.