To rule out the possibility of our old outdated mirrors serving an old outdated keyring, could you determine which of the mirrors are from OSDN and remove them?
cat /etc/pacman.d/reborn-mirrorlist | grep -i osdn
Then edit the file to remove all those lines:
sudo nano /etc/pacman.d/reborn-mirrorlist
Save using Ctrl X
Then you can try an upgrade normally
sudo pacman -Syyuv