Does macOS support installing and boot from a single volume using SoftRAID?
Yes, you must have a dedicated disk as a backup boot drive now.
One idea is using Carbon Copy Cloner to make the second copy a "disk image", so you have a restorable system. CCC knows how to handle these situatons, and is very good for that.
Users can get Apple RAID to boot Mojave by cloning. (sometimes SoftRAID volumes, but it is tricky and I do not have a realiable method to give you)
Catalina, I think it is very difficult to get an Apple RAID to boot, if not impossible. I have not spent time trying, however.
Apple security team won this battle. The system is going to be even further locked down with ARM. As it is, you cannot remove an internal hard drive from one computer and put it in another. the newer ARM macs will be more like iPhones in their flexibility.
That does not sound exciting... that would turn Macs into consumables... that is quite the thought to digest.
That is what I predict. There will be the "Mac Pro" models, but everything else will become consumables.