The libraries are completely open. There are 3 types of licenses - Qt Commercial (allows using qt libraries to make closed source applications), lgpl (2.1 with exceptions and 3.0) and gpl (2 & 3).
The KDE team has not moved away from Qt since the troll tech days because there is no need to move away from L/GPL libraries.
The bit about it “not being open source” is because Qt had its own Qt Free Edition License back in the 90’s which the FSF decided at one point that it didn’t meet their definition of free.
It then went QPL with a foundation that guaranteed a BSD style license if anything happened (like being bought out).
In 2000 they went GPL.
Linux Mint Debian Edition is my standard recommendation for desktop for those newer folks.
Straight up Debian for everything else. Debian is my desktop. And all of my servers (aside from some things I’m testing for work or something where I need to test against RHEL or something).
And Proxmox for VMs.