Manjaro cannot change how packagekit works. Its up to you or your system administrator to decide whether it is included with the system.
What? Its an entirely optional package.
No, you do not need it.
No, important things will not break without it.
discover deepin-store and apper all use it for package management … but none of those things are considered safe. (read as - do not use any of those things, or packagekit in any way for package management. Just dont.)
Actually, in the case of discover … it is an optional dep - discover can be installed and used for its other functions including fwupd and addons from store.kde.org.