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.