I finally received a reply to my Apple bug report about MacOS 10.13 not installing to Apple RAID volumes created or in use with MacOS 10.12 even with OEM Apple hardware. This may affect how some people manage FMS system upgrades in the near future. I have two Mac Mini Servers that run Apple RAID1 mirrors on the OS volume that are impacted by this change.
"...we stopped allowing installs to RAID set members in macOS 10.13"
I have heard that some people have succeeded in installing 10.13 on SoftRAID volumes.