Here is the console output $ systemsettings5 file:///usr/share/kpackage/genericqml/org.kde.systemsettings.sidebar/contents/ui/main.qml:31:5: Type CategoriesPage unavailable CategoriesPage { ^ file:///usr/share/kpackage/genericqml/org.kde.systemsettings.sidebar/contents/ui/CategoriesPage.qml:113:5: Type Kirigami.Heading unavailable Kirigami.Heading { ^ file:///usr/lib/x86_64-linux-gnu/qt5/qml/org/kde/kirigami.2/Heading.qml:50:1: Type QQC2.Label unavailable QQC2.Label { ^ file:///usr/lib/x86_64-linux-gnu/qt5/qml/QtQuick/Controls.2/org.kde.desktop/Label.qml:27:1: module "org.kde.kirigami" version 2.2 is not installed import org.kde.kirigami 2.2 as Kirigami ^ [1] 11778 segmentation fault systemsettings5 Modules seem to load fine using kcmshell directly, though.
The new System Settings needs Kirigami 2.2, and your system doesn't seem to have it, which is why it crashes on launch. This would be either an issue with your distro (not marking the dependency correctly or providing an updated package) or on your own system (perhaps the upgrade to the Kirigami 2.2 package failed due to a dependency conflict?). Either way, this is not a KDE issue.
Thank you for that, it was an issue of version mismatch between Ubuntu repo's qml-module-org-kde-kirigami2 (which has kirigami 2.0 but version tag 5.38.0-0ubuntu1) and Neon's.