Installing using http://cdimage.ubuntu.com/ubuntu-server/bionic/daily/20180525/ d-i server iso, I can see that raid1 is offered as an automatically partition drive. The text name of it, is a bit ugly, but it is there. Package version in use there is /pool/main/m/mdadm/mdadm-udeb_4.0-2ubuntu1.1_amd64.udeb and there are symlinks to the raid device present in /dev/md.
Booting with http:// cdimage. ubuntu. com/ubuntu- server/ bionic/ daily/20180524/ which has /pool/main/ m/mdadm/ mdadm-udeb_ 4.0-2ubuntu1_ amd64.udeb there are no symlinks present in /dev/md for my raid1 device.
Installing using http:// cdimage. ubuntu. com/ubuntu- server/ bionic/ daily/20180525/ d-i server iso, I can see that raid1 is offered as an automatically partition drive. The text name of it, is a bit ugly, but it is there. Package version in use there is /pool/main/ m/mdadm/ mdadm-udeb_ 4.0-2ubuntu1. 1_amd64. udeb and there are symlinks to the raid device present in /dev/md.