Yahoo is introducing a new mail version to most users over the next couple of days, the missing "notepad" feature is related to that. This does NOT mean that Yahoo will do away with it, it's just not available anymore from the lists of features. What works for now though is this direct link, maybe bookmark it, just in case:
http://notepad.yahoo.com/
http://ycorpblog.com/2012/12/11/introduc…