I also seem to rember msn messenger also had a silent blocking feature. Your activity would be invisible to the blocked user and you'd appear as offline.
Ignoring a user is entirely client-side. You might be able to test it with a ctcp, depending how the client is designed and configured, and depending on their umodes (if ctcps are blocked) as ctcps are standard messages with a prefix.
Silencing is unlike Reddit shadow bans; firstly, it only applies to private messages, not to channels. Secondly, it has to be configured by the individual users, not by admins.