Two years later, I can say that this error might still be a thing.
I concluded that the most likely cause is my mother. Jokes aside, she unplugged the power
chord before the pc switches off completely. Through the years, she managed to crash in
the same way f33, f34, and now f35.
This time I had a look at the hard drive, and I can try to add something to the
discussion.
The situation is the same as described above, kernel panic at switch-root. Three kernels
were available (up to 5.12), and they all failed in the same way. Partitions look fine,
fat for UEFI (not tested, but hey, I reach boot without problems), ext4 for boot, btrfs
for data (2 sub-volumes with / and /home mount points) without encryption. All btrfs
checks run from a live USB key were perfectly clean.
I tried to switch root from the live key, and... error.
Some executables and libraries were completely deleted (zero sizes). I have a partial
list, but I don't think it will be useful. Core software was all there except for
python.
Unfortunately, I already reinstalled f35 by nuking the root subvolume and creating a new
one. Next time I will be smarter and save it for further analysis.