Sixty or more years ago I read a humorous story - possibly by A.A. Milne - in which the two main characters were called ETAION and SHRDLU. Later I came to realise that these names were taken from a letter frequency list There are a number of these lists with small differences between them reflecting the context of the letter count. For Language on Vacation Dmitri Borgmann used one starting ETAISONHRDLU, differing from my author's only in the positioning of the S. This article is concerned with letters in crosswords, but it starts from the other end by looking at letters not present in their solutions.
"Omitted Letters in Crosswords,"
Word Ways: Vol. 39
, Article 9.
Available at: https://digitalcommons.butler.edu/wordways/vol39/iss2/9