Via Daring Fireball, cricket, a former Apple employee, recognized State Manager as a project he worked on for the Mac called shrinkydink:
Both [shrinkydink and Stage Manager] took over the positioning and size of all windows. Back then, we saw this as a huge win for novice to intermediate users who struggle with window management.
Stage Manager appears to be positioned as a power-user feature which I think is a shame. I’d much prefer to see it as something you pick in Setup Assistant or choose in Settings rather than hidden in a menu somewhere. I think this is something that would be especially appealing to a new Mac user. On the iPad, I don’t see any reason to use the existing multi-tasking UI anymore.
So the power user feature we’ve been waiting for on iPadOS–window management–was conceived as a novice user affordance. I remain hopeful, but this wasn’t exactly what I was hoping for.