The CMS UI update described in this post was originally released as CMS UI 11.21.0. Due to unintended semantic breaking changes, CMS UI 11.21.0 has been remvoed from the Episerver Nuget feed and will re-released in the future with some additional bug fixes. The current suggestion is to downgrade to CMS UI 11.20.x for now.
The latest version of the Episerver UI comes with a new look which changes up how things work and is intended to create a consistent look/feel across all parts of the Episerver platform. The UI update as described in the CMS UI refresh post and is designed to be an opt in so for any existing modules the UI should remain the same.
The blog describes how I updated Virtual Template System for Episerver to be compatible with the new Episerver Platform UI.