Comment 1 for bug 1966591

Thank you for taking the time to report this bug while providing a good reproducer.

I was able to reproduce in Focal and Bionic

# ssh-keyscan github.com >test_known_hosts
# ls -la test_known_hosts

-rw-r--r-- 1 root root 656 Mar 28 14:24 test_known_hosts

# ssh-keygen -R github.com -f test_known_hosts

test_known_hosts updated.
Original contents retained as test_known_hosts.old

# ls -la test_known_hosts
-rw------- 1 root root 0 Mar 28 14:25 test_known_hosts

However in Jammy and Impish this is fixed:

# ssh-keyscan github.com >test_known_hosts
# ls -la test_known_hosts

-rw-r--r-- 1 root root 656 Mar 28 14:30 test_known_hosts

# ssh-keygen -R github.com -f test_known_hosts

test_known_hosts updated.
Original contents retained as test_known_hosts.old

# ls -la test_known_hosts
-rw-r--r-- 1 root root 0 Mar 28 14:31 test_known_hosts

With this already being fixed in the newer releases it should just be a matter of finding the relevant commit and adding it to focal and bionic.

Thanks