It is not a patch. It doesn't require any code changes to OpenSSH.
opkssh uses AuthorizedKeysCommand field in sshd_config. OpenBSD added this config field to OpenSSH to enable people to do stuff like opkssh or instance-connect without needing to do code patches. OpenSSH is really smart about enabling functionality like this via the config.
At least that is my belief, do people here think my speculation is correct ? I checked https://undeadly.org and no mention of anything like this.
FWIW, I will never use this.