Thank you, this is interesting to read. I also use ISMLP from time to time and can only imagine how valuable it is to actual musicians. Now, it is simply true that sheet music that is under copyright is, indeed, under copyright, but as ISMLP focuses on classical music it’s not such a big deal, as much of it is in public domain (many 20th century classics still aren’t, I believe, such as Stravinsky, Shostakovich…), at least the original old editions.

just take a look at this short list taken from your own list of “banned books” affected by the decision:

“The Adventures of Huckleberry Finn” by Mark Twain (first published 1884-85) “The Awakening” by Kate Chopin (first published 1899) “An American Tragedy” by Theodore Dreiser (first published 1925) “Candide” by Voltaire (first published in 1759, also in English translation, again in English 1762) “The Decameron” by Giovanni Bocaccio (written ca.1353, published in English by 1620)

All five of the originals are public domain worldwide, even the two translated into English.

This part lacks important detail, though. The two translations are likely to be new ones, not from 17th/18th century, so they have new copyright too. The other two books may be under legitimate new copyright because of the supplementary materials or textological work. I talked about this with some people on reddit who I guess were knowledgeable about this, and basically when an editor works on a new edition they might introduce corrections to the text based on the manuscripts or some other version of the text (e.g. censored sections). This is work that should (I guess) also be copyrighted. Now, I haven’t gotten a completely satisfying answer about what really can be covered by this, because it can be difficult to explain whether mere modification of spelling of e.g. Shakespeare (original <walk’d> = modern <walked>) counts as copyrightable work, or does it require more extensive work (such as dealing with the textual variations in early Shakespeare editions, which are mind-boggling).