Sony has made its so-called Small Apps a fixture on devices going all the way back to the Xperia T in 2012. For the unaware,they are floating apps that can provide quick access to things like email, a calculator, timer, and so on. On the Xperia X line of devices, Small Apps are no more.
On older devices, Small Apps were accessible from a favorites bar at the bottom of the multitasking screen. However, that's missing on the Xperia X lineup, and there does not appear to be any way to enable it. A community manager on Sony's mobile forums claims the change was made because, "This is according to the specification, and how Google has defined it." It's not clear what exactly Sony is referring to here.
Small Apps weren't necessarily a bad idea, but they never caught on. There are only a handful of supported third-party apps in the Play Store. It's even less robust than Samsung's Pop-up view. The change will probably go mostly unnoticed, except by Sony fans who used Small Apps heavily. I'm sure there are some.