Shouldn't your terminal handle this even if you were using netcat to chat? If the client was injecting newlines that would end up being rather unpleasant when pasting chats into a different channel.

That would prevent indenting the messages to the length of the nickname. Irssi performs manual linewrapping to achieve that.

Who uses netcat to chat?

Not only it injects newlines, it injects indentation. You can always 1) paste from logs 2) paste from the in-memory rawlog Irssi keeps or 3) use a script like xsel | tr '\t' ' ' | tr '\n' ' ' | sed 's/ */ /g' | tee >(xsel -i) | xsel -ib to your clipboard (yes it uses tr uselessly)

Copy-pasteability is not really the goal of apps that use ncurses or similar interfaces. Side-by-side windows break it also.