Um... Why in the world would you do that? Strikethrough is normally not used to edit someone else's message except in a shared document type of situation where you want to keep track of who did what.
Anyhow, I'm not saying it can't be used. I'm just saying that it really doesn't have a real use other than "just because." There are many more things I'd rather see implemented than something that doesn't have a good use for it. And because the majority of reasons people use strikethrough elsewhere don't really apply to IRC, I think that even if it's added to the "todo" list that it is placed at the bottom.
