This will ruin compatibility with other clients, and imho it's useless. The reason strike-through was invented was so you can "update" text, i.e. you say "the party will take place on Friday" then it gets changed to Saturday so you strike Friday, and change it to Saturday. What use does that have in IRC? You can't update messages you've already sent to the server, so you'd be doing something like "the party will take place on <strike>Friday</strike> Saturday" in which case, why bother? The only place I could see it being remotely useful is in a channel topic, but again it breaks compatibility and I don't think it should be added.