STATUSMSG is a type of IRC private message / notice where the target of the private message is the members of a channel with at least a certain level of channel responsibility.
In other words, instead of the target of the message just being #ircv3, it may be @#ircv3 and only be send to the ops of the channel, rather than all members of the channel.
The STATUSMSG RPL_ISUPPORT token is used to note that the server supports sending private messages and notices with these special 'statusmsg targets' (the channel name prefixed by a given privilege level character such as +, %, @ or similar).