Arjen Hiemstra has published an article on the status of the Union project: a single system to support all of KDE's technologies used for styling applications.
The work on Union's Breeze implementation has progressed to the point where it is very hard to distinguish whether or not you are running the Union version. We have also tested with a bunch of applications and made sure that any differences were fixed. So we are at a stage where we need to get Union into the hands of more people, both to get extra people testing whether there are any major issues, but also to have interested people creating new styles.
This means that with the upcoming Plasma 6.7 release, we plan to include Union. Discussion is currently ongoing whether we will enable it by default, but even if not there will be a way to try it out.
See Hiemstra's introductory article on Union, published in February 2025, for more about the project and its creation. KDE 6.7 is expected to be released in mid-June.
Instagram Engineering
Shopify Engineering