Adobe just released version 21 of the Digital Publishing Suite for everybody using InDesign to create digital magazines. As always the Folio Builder Panel is mandatory to update, you can keep your producer tools at the version you like.
If your Folio Builder Panel is telling you to update, the link inside the message is wrong (Anselm reported this morning). Make sure to grab the latest version after log in at digitalpublishing.acrobat.com.
The most important change in this release is the ability to set the folio version upon creation of the folio. With this you can create a folio that is compatible with a viewer down to v20 although you have the recent producer tools installed.
Since the Adobe Viewer v21 is currently in review at Apple, you can create a folio in v20 to be compatible with the current available viewer or if your live app you are creating for is made with an earlier version of the tools. You cannot create a folio for an earlier version than 20.
Other changes in v21:
Preview the current layout on the iPad or iPhone
For iOS users, it is possible to send just the current article to the iPad or iPhone for preview. Previously, you could only preview whole folios. This makes it a lot faster to preview your content on the device. The article does not require to be included in a folio.
Improved Social Sharing
If you create your folio in PNG format, the Web Viewer is handling interactive overlays a lot better now. If you create a PDF folio, the viewer in the desktop browser will still show PDFs of your article.
Auto-View during Download
When you buy or download a new folio, the viewer immediatly opens it when content is available.
Subtle texture in browse mode
Any viewer from v21 will show a subtle texture in the background of browse mode.
A lot of other improvements are on the backend side this time, so there are not many improvements for authoring in InDesign except of the versioning feature.