Just adding my ¢2:
On more recent systems, unless you’ve changed the default behavior, journald shouldn’t actually be logging to disk: see Change in logging behaviour on Ubuntu Core.
At https://gist.github.com/JPvRiel/b7c185833da32631fa6ce65b40836887 there’s a nice overview of how to enable/disable persistent journal storage. As that article says, even with persistent storage enabled there should be “sane” defaults for SystemMaxUse and SystemKeepFree of 10% and 15% respectively. I’ve experienced these settings to work as expected in practice.
Even more broadly than Ubuntu Core, I’d say that this is a Linux system question: how do you ensure that there is always enough space left for “really critical” services to run, when other “maybe also critical” processes are also competing for that space? If anyone has come across effective solutions to this issue, I’d love to hear them - though maybe that’s a discussion for a separate thread.
Measures such as reserved filesystem space, or user quotas, are probably not adequate on a system where many of these processes are running as root anyway (but maybe some segregation is still possible?). In days past, some adequate segregation may have been achieved through disk partitioning, but that hardly seems like the right solution on contemporary systems.
Either way, ensuring that individual processes like snapd are conscious of their own space use is a good step forward!