Ideally, if the user is rebooting explicitly to install updates (e.g. clicking “Restart & Update” from the dialog in GNOME Software or KDE Discover), performkexec twice instead of reboot:
Download updates ----> kexec into system update mode ----> Install updates ----> kexec updated system/kernel.
kexec is great, but has one significant shortcoming: some hardware and/or drivers do not handle kexec properly. In particular, various graphics drivers had this issue that after a kexec apparently some state was carried over and the display was garbled. Because of such shortcomings, kexec was never the default. But if you test it and it works properly on your machine, then great, by all means use it. It is also possible that the quality of hardware, firmware, and drivers has improved, and we could recommend it more widely, but we would need to back that up by widespread testing.
We could probably expose an easy way to toggle it on and off?
kexec works mostly fine in a server fleet but of course those tend not to have GPUs (and the hardware is much less heterogeneous so can be better tested)
I think we could expose this as an option in /etc/systemd/sleep.conf. We configure various aspects of how suspend and hiberation works there, so also controlling details of reboot would be a natural fit.