I’ve commented here and there about the BSD foundation underneath Apple Time Capsules and other AirPort devices.
Via OSNews, I learned of an interesting hack that purports to allow Time Capsules to run “modern” Time Machine backups. This would enable Time Capsules to perform Time Machine backups with macOS 27 clients and later by adding a SMB3-compatible server to them, albeit with some caveats.
I haven’t tried this trick, and I cannot endorse it. Indeed, the trick demonstrates how easily someone with a little bit of Unix knowledge and local access to a Time Capsule can thoroughly compromise it.
Instead, I strongly recommend retiring any Time Capsule from use and upgrading to a modern, currently supported alternative at your earliest convenience. Nonetheless, some may find the “TimeCapsuleSMB” method to be useful, interesting, and perhaps even a little frightening.
PS. See also this Apple Insider post.