Changing the sshd_config I made a mistake mis-spelling "yes" as "Yes".
As a result "systemctl reload sshd.service" killed the SSH connection I was
using to manage the server.
As I understand it, without systemd the active SSH connection would not be
killed, leaving a chance to fix things. However with systemd, every sshd
process is killed when the main sshd terminates (it seems).

More details can be found at https://bugzilla.mindrot.org/show_bug.cgi?id=3558