I think it would be helpful to have some sort of message in the area where Silent Post is displayed in CBsubs that says whether or not the Silent Post URL is working or not - similar to how Acymailing will tell you the last reported connection details of their cron. It is really important that this is working and there's really no way to tell until well after there's a problem. Plus, a lot of us are using things like RSfirewall or Cloudflare which can interfere.
CB 2.3, CBsubs 4.3, PHP 7.1, J! 3.9.X