The KDE project and the open-source Linux community have found themselves in a fix after The Qt Company dropped the Qt 5.15 LTS release from the open-source release, making it only available to its commercial users. This means they have resorted to maintaining their own Qt 5 patches.
There are several modules and other features still missing from the Qt 6 toolkit in addition to most of the free software.
The KDE project has taken up maintaining their own Qt 5.15 release at least until they have fully transitioned to using the Qt 6 toolkit.
Read more about the move from the announcement about the Qt 5 Patches maintenance from this post here.