I use PCLinuxOS as my primary Linux OS. They are a bit conservative to adapt new updates until they are sure of stability because of rolling nature. KDE is still at 5 there. Heard about Neon and wanted to try KDE 6. I find that they have adopted Windows style approach to updates where we need to reboot to apply the updates and we cannot do anything on the system while the updates are processing. Recently managed to install Fedora because I heard that their EFI is secured or something and can survive clobbering by Windows updates in a dual boot setup. But they also seem to have gone with offline updates.
Are offline updates necessary due to them using SystemD or is the Linux world in general moving in this direction?
It’s not systemd’s fault, though systemd most often implements offline updates. The arguments for and against offline updates have nothing to do with systemd.
A lot of Linux distros, and graphical package managers like Discover and GNOME Software, are moving in that direction, under the argument that updating while online can cause disruptions to running software, in the worst case including the package manager itself (which can brick the system if it occurs in the middle of a critical update), and updates can’t be applied until the affected program (or the system, in case of critical components like the kernel) restarts anyway. Fedora Magazine explains the reasoning here: https://fedoramagazine.org/offline-updates-and-fedora-35/
In my personal experience though, I have never had an issue enabling automatic online updates on Debian Stable, and have had computers stay online for several months without any noticeable issues beyond Firefox restarting, so the risk is there but it’s pretty minor.