For starters, it’s not a kernel problem, so booting an older kernel wouldn’t help. It’s a problem with PackageKit and/or GNOME Software doing stupid things.
Today, there were two packages meant to migrate to byzantium-updates: librem5-base and linux-librem5. They both need each other in their latest versions and break the older versions of each other. However, only one of them has migrated.
This isn’t a problem on its own because older versions are still in byzantium repo, and apt is smart enough to figure out that one of them can’t be updated on their own:
$ sudo apt full-upgrade
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
Calculating upgrade... Done
The following packages have been kept back:
linux-image-6.3.0-1-librem5 linux-image-librem5
0 upgraded, 0 newly installed, 0 to remove and 2 not upgraded.
However, looks like either PackageKit (used by GNOME Software) or GNOME Software itself is not, and tries to install the new kernel package anyway. Since it has a “Breaks” relation with older (still installed) librem5-base, it decides to resolve that conflict by… removing it.
Until it’s resolved, please don’t try to upgrade with PureOS Store. Upgrading with apt is fine (unless you explicitly tell it to do stupid things by yourself). The next migration is supposed to happen in two hours, so hopefully it will be resolved by then.
In case you already did the upgrade and broken you system, you need to do:
sudo apt install librem5-gnome
In case it fails to resolve the dependencies (and only then!), do this instead:
sudo apt install librem5-gnome linux-image-librem5/byzantium linux-image-6.3.0-1-librem5-
I understand that the challenge will be to get a shell access in order to execute that in the first place though.