From http://build.neon.kde.org/job/xenial_unstable_plasma_kscreenlocker_bin_amd64/70/consoleFull: 04:31:35 -- The following OPTIONAL packages have not been found: 04:31:35 04:31:35 * Seccomp , The enhanced seccomp library. , <https://github.com/seccomp/libseccomp> 04:31:35 Used for putting the look'n'feel package in the greeter into a sandbox. 04:31:35 This is an important security feature which got pre-announced on distributions mailing list.
https://packaging.neon.kde.org/plasma/kscreenlocker.git/commit/?id=7947587d5dadf66f070ca34f42171363d3f38d02