These kind of announcements for large moves have historically been done on G+, but with the shutting down of G+, better start announcing somewhere different too. A copy this time of https://plus.google.com/u/0/115793168478540780255/posts/BpvtiGWEDah

Qt 5.12.1 has been in the [build] repository for the last 9 days. All of KDE needed to be build against it, and now that Frameworks 5.55.0 is released, all can move to the stable repositories. This means not just all of Qt5 and Frameworks, but also the KDE Apps 18.12.2 released last Thursday have moved.
That is not all though for this update, a very large rebuild was also done for the move to Readline 8.0.0, thus in all well over 500 packages have just moved.
Another note to make, Libreoffice 6.2.0 has entered the build repository 2 days ago as a pure Qt/kf5 application, please help with testing if you are on [build], so we can be sure this change is not causing any regressions before it moves to all users.
https://resources.qt.io/qt-blog/qt-5-12-1-released
https://www.kde.org/announcements/kde-frameworks-5.55.0.php
https://www.kde.org/announcements/announce-applications-18.12.2.php
https://wiki.documentfoundation.org/ReleaseNotes/6.2

13 days later
demm changed the title to Qt 5.12.1, Readline 8.0.0, Frameworks 5.55.0 and KDE Apps 18.12.2 .