Glass can do this.
kind of a richard move to kick people out as they join
badspot has banned people from the master server for things like this, like hosting older versions of blockland
there is no good way to do this, the best user experience is to tell them that they're missing things and provide a link, but let them play without it
what may work better than to kick them is to password the server, and the addon automatically shows it as no password + puts in the correct password
Not really.
No one has been master banned for requiring certain add-ons. Only for flooding, faking information, and running older versions.
And letting people run around not getting the full experience is probably the worst idea. Especially when they're also taking up space and potentially harming the experience of players who are invested.
The password thing would take a lot of work, and would only make the gamemode harder to pass onto other people when the host is done with it.