Comment 31 for bug 44609

Desktop images do have mdadm and lvm2 packages installed, and it does have support to set up full disk encryption and lvm2. Installing RAID on typical desktop configurations is uncommon. But you can proceed to do so by manually assembling RAID devices before starting ubiquity in "Try Ubuntu" session. Do mind that you will need to chroot into target at the end of installation to install mdadm & update initramfs. This can be automated / scripted.

Do note that ubiquity installer had fakeraid support for a long time, and those do get auto-assembled on boot.