Following is the text that Jim Pate sent me in answer to a question about why I couldn't find ZEK and other words in any of the dictionaries last used to update the OWL.

In addition to the referral link that John Chew gave you I would also note that with each new edition of each source dictionary (and even with each separate printing of each edition) there are usually a number of additions and deletions of words that are made by the dictionary publishers for different editorial, lexicographical, and marketing reasons.  The evidence that we have is that the two-letter and three-letter words you cited were in at least one edition of one of the source dictionaries used in the past.

Your assumption that these were grandfathered in is correct.  While there is no specific written policy regarding when or if these and other words that are good now but will be eventually removed from the source dictionaries would be deleted from the official lexicon, the de facto policy is one of removals only in the case of actual errors of inclusion in the first place.

If you carefully consider the situation, even with the removal of the small number of errors that have occurred in the past -- particularly with the shorter words, it is very difficult for Scrabble Crossword Game players of long standing to completely expunge the removed words from memory.  If you speak with a large number of players who have been in clubs and tournaments throughout the various editions of the Official Scrabble Players Dictionary I suspect that you would find that an extremely high percentage of these would have stories of mistakenly playing DA or VIN or one of the other words deemed errors that were later taken out of the lexicon.

Additionally, ask yourself the following question.  "Why would a word that was perfectly good in 1979 and that might be seen in printed material both before and after that date now be any less of a word just because none of the source dictionaries currently in print continue to list the word for various reasons?"

I hope that this additional information is helpful to you.

Sincerely,
Jim Pate
Chair, NASPA Dictionary Committee