For deployments that have https://hg.prosody.im/trunk/file/tip/plugins/muc/register.lib.lua#l7 and use https://modules.prosody.im/mod_muc_http_auth.html users can still register to a MUC even if they are not allowed to join. That means they would receive RAI or RMN, for instance.

module:hook("message/host", function (event)
	local stanza = event.stanza;
	if #stanza.tags == 1 and stanza.tags[1].attr.xmlns == "http://jabber.org/protocol/chatstates" then
		return true;
	end
end, -10);