Well, see, I guess the difference is that I don't see it as all that unclear. Willow/Tara was slash until it became canon, and then it was not slash any more, at all, because the characters became gay/bi retroactive to that point in canon, and therefore also retroactive in fic.

The original definition of slash can be easily applied to any scenario to parcel out what is slash and what isn't, unless you want to make it difficult. Just because a story sucks does not mean it's not slash, for example, though it might feel better to punish it for sucking by not allowing it to be categorized with the same terminology as the (subjectively) non-sucky slash stories. And, ignorance is no excuse. Whether or not a reader knows about the canon doesn't change the canon; applying a label unilaterally with no reference to reality doesn't make the reader right, it just makes her uninformed.

I don't deny that uninformed people seeking to make things more difficult than they need to be alter ideas and terminologies on a regular basis. I just don't intend to do that with this word, in my own worldview.

*goes to bed*