I think the /seen plug-in is actually different across gamemodes. Some provide more information, anyways support support
The reason you can no longer see if a player is banned through /seen is because we have a new ban plugin. We used to ban through essentials (which is why /seen was compatible), but this new plugin is not connected to essentials in any way. So, I'm not too sure if it's possible to add back. This is only the case for lobby bans. As said above, we do have a new ban plugin in which still has some little bugs in it. I definitely support the fix of it. The head mods are aware of it, so we're hoping for proper lobby ban messages soon.