That's not it. Plasma 5.10 is installed and latest version:
apt-cache policy plasma-desktop
plasma-desktop:
Installed: 4:5.10.5-2
Candidate: 4:5.10.5-2
Version table:
*** 4:5.10.5-2 500
500 http://deb.debian.org/debian unstable/main amd64 Packages
100 /var/lib/dpkg/status
Other installed plasma* packages are also at 5.10.5-2 . If you look at my paste log, all plasma packages are listed to remove but not replaced.