Client state indication helps for things like suppressing presence updates, but it doesn't go as far as to e.g. let you only be notified for messages that would highlight you in a large MUC, right? So either you're joined to the MUC and receive all messages in it, or you leave the MUC and don't get notified about any of them.
Although now that I read the spec again, it seems general enough that maybe it could be used to implement behavior like that? I don't think I've seen any servers that do, though.
Prosody has an option for filtering MUC messages with mod_csi_battery_saver (which is the currently recommended community module for CSI), I don't think ejabberd does though.
> groupchat messages must set a subject or have the user’s username or nickname mentioned in the messages (or be encrypted) to count as “important”.
Although now that I read the spec again, it seems general enough that maybe it could be used to implement behavior like that? I don't think I've seen any servers that do, though.