You can find more about it here: Kirby 5 | Kirby CMS
We’ve built a brand new architecture to store different versions of a page. This allows us to finally preview changes properly in rendered templates. But we’ve decided to start with a simple setup with two versions: the latest published state of a page and the changed state of a page. The editing experience in the panel feels pretty much identical to what we had in v4, but with a shiny new preview mode. Under the hood, a lot of new stuff is happening though. Changes are no longer stored in the browser in localStorage, but on your disk in a new _changes subfolder. This will allow us to introduce multiple versions of a page in a later state and come up with something like revisions or multiple states of a page.