• Could be the kernel itself

      Wouldn’t make sense to me because the thread says GNU/Linux and others, though this could relate to Android or distros not using any GNU.

      gnupg

      Usually not exposed to the network though, but it’s generally a mess so wouldn’t be too surprising

      Another candidate I have in mind is ntpd, but again that is usually not easily accessible from outside and not used everywhere, as stuff like systemd-timesyncd exists.

      Just want to stress that I’m not sure about it being OpenSSH, it was more supposed to be a fun guess than a certain prediction