It turns out, the boot ID is a thing. Running hostnamectl shows a unique “Boot ID” that’s changed each reboot.
Running journalctl --list-boots gives you the boot ID of every past session (as long as persistent journal logs are enabled).
It seems like this is useful for getting the logs of specific past sessions as you can use journalctl --boot=id.
I’m not sure how this can be set to the same for each boot session for every Whonix user or even if it’s worth doing.