In it, you can see the app switcher adopting a more iPad-style layout by fitting four active app snapshots on one display at a time, as opposed to the current one. In addition, there is an apparent lock type functionality that prevents you from swiping to close an app that you don't want to be accidentally disposed of.
The new iPhone app switcher preview is reportedly not achieved via some sort of a jailbreak mod, too, so what we are seeing below might indeed be how Apple will approach app switching on all of its iOS devices going forward. Alternatively, it may simply reserve this iPad-style switcher for the big Max models, and leave the upcoming 5.4" iPhone 12 alone in that four-apps-per-screen respect.
Does Apple have other multitasking shenanigans up its iOS 14 sleeve? Well, the WWDC expo is usually held in June, and there will be quite a lot of leaks and previews before than, so this fancy iPhone experiment in app switching may fall off the pipeline at some point before the first iOS 14 public beta is released.