Right now High Sierra doesn't support AppleRAID, unfortunately, at least not on the boot volume (even with HDs, not SSDs). I have the same frustration, because I use AppleRAID RAID1 extensively on my Mac servers (with SMARTReporter to report via email if the RAID or drives have an issue).
I would suggest just staying at Sierra 10.12 until High Sierra 10.13 or a subsequent version brings back support for AppleRAIDs...if it ever does.
If you need to update to High Sierra right away for some reason, or if Apple never comes through with AppleRAID support in High Sierra*, the alternative is SoftRAID, with both full ($179) and lite ($49) versions available, which provides excellent software RAIDs on all versions of macOS, including High Sierra. I use it for RAID5 software RAIDs and it can also do RAID1.
One unfortunate limitation of SoftRAID Lite is that it won't send email notifications for RAID issues, which makes it a non-starter for me, so if you want notifications then you end up paying $179 for the full version to create a RAID1 that was formerly free from Apple. FWIW, however, SoftRAID does have a ton of features, including proactively checking and validating your drives.
* Apple has been seriously curtailing things that I've depended on for years, like AppleRAID and macOS Server (both the hardware and the software). The latest insult is this nonsense about deprecating all the best features of Server, so prepare to look for alternatives for doing what you're doing with Server in the not-so-far future. For webserving, Apple's suggested alternatives are a PITA (IMO), so I'm looking at alternatives like Abyss. For SMTP serving, Postfix with one of the add-ons for management might not suck at ease-of-casual-management too much (although I'm not getting my hopes up). Sigh...
