Let me comment that the text of the article does not suggest that there are no more than eighteen such words, thus "which" is selective and appropriate.
The words that should not have gone out of style are a subclass of the words that have gone out of style.
This is not correct.
The words that should not have gone out of style are is a subclass of the words that have gone out of style.
Which words are we talking about? We are talking about 18 of the words which (referring to the words)(selective, plural) form a subclass that (referring to the subclass unit)(demonstrative, singular) ought not to have become obsolete.
Furthermore, from Webster's Third New International Dictionary Unabridged (1981):
A. The definition of "which" that applies: (grin)
Second example (excerpt):
which - pronoun - used as a function word to introduce a restrictive or non-restrictive relative clause and to serve as a substitute within that clause for the substantive modified by that clause; used in any grammatical relation within the relative clause except that of a possessive; used esp. in reference to animals, inanimate objects, groups, or ideas
B. The definition of "that" which applies: (big grin)
Fourth example:
that - pronoun - no plural sense - (1a) used as a function word used to introduce a restrictive relative clause and to serve as a substitute within that clause for the substantive modified by that clause; used in any grammatical relation within the relative clause except that of a possessive or the object of a preceding preposition
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As the article is given, the lack of the definite article before the substantive "18 words" clearly announces that this class is not restrictive--there may be more words (and there definitely are more) than those selected. Therefore, here the definition of "which" applies, and that for "that" does not.
Capisce?
You have made a good case. Agreed. Thanks.