Interesting detail about Square that I learnt on Twitter shortly after SwiftUI was launched...
Square sold iPads with their software on as "appliances", so they can't deprecate them on a schedule like most software. For this reason the Square iOS codebase (which is a single codebase) must support back as far as iOS 9.
Because of this, they're considering an internal implementation of SwiftUI so that they can start using it now, rather than in ~5 years time. I wouldn't be surprised if they open sourced this if they do go ahead with it.