TBH I have never used software (aka "fake") RAID (actually I've never used RAID fullstop). I have heard multiple arguments for and against it. E.g. ProxmoxVE refuse to include/implement it and also refuse to provide any support to anyone who uses it as they believe it to be inadequate, unreliable and provides minimal benefit. OTOH I have heard many reports of people who use it and are very happy with it. IIRC Liraz uses it on his computer...

I have also heard horror stories about people who have used hardware RAID (as advocated by Proxmox devs as the superior and only reliable RAID option) and the problems that can be encountered if/when your (expensive) hardware RAID controller dies. That's because you need the same hardware controller to rebuild your array; whereas with software RAID it doesn't matter what hardware you have. So bottom line is it seems to be quite a contentious subject...!

So with that perspective in mind I'm not sure that we'd want to actually include it by default (I'd argue that inclusion is essentially endorsement). Regardless of that it would only make sense in the ISO builds (all other builds are for varying types of virtualisation) so even if it were included it would make sense to remove it for all other builds (in buildtasks).

However at the end of the day we want people to be free to use TurnKey as they desire. We also want to make it as easy as possible and as newb friendly as possible. So I think that at the least providing some info would be a great thing!

With all the above in mind my suggestion would be to A) simply document how to do it (you probably already know that the turnkey docs are a wiki that you can edit/add to - I'd suggest in the "how to/tutorial" section). And/or perhaps it would make a cool blog post?! And/or B) make a helper script that sets it up (and probably stick it in your github repo).