ssh keys get stored in my encrypted keepassxc/keepassdx database. Per: Password management 2019 check-in - #2 by trashHeap
The encrypted database itself is mirrored to multiple machines for redundancy. (Previously via syncthing, now by NextCloud.). Nextcloud data and the various device filesystems are encrypted. For doubly encrypted safety. Nextcloud has versioning in place to protect against synchronizing some corruption to the database. Nextcloud is also periodically backed up by BorgBackup. (again encrypted).
Once a year (usually shortly after new years) I make a plaintext backup of the database to a thumb drive and I store that in a safe,secure and fire-proof place.