Writing a "good" book introduction is a difficult thing--not least because of the fact that readers do not agree among themselves on exactly what it is they are looking for in an introduction. Some readers are looking for information about the life and career(s) of the author(s) while others are hoping that the introduction will place the book into a larger context. Some readers think that the larger context the book should be placed into is that of the author(s) thematic and stylistic growth (and perhaps decline.) Other readers think that books should be placed within the context of the time and culture in which they were initially written and/or published. Yet other readers would prefer that books were placed within the context of other books written/published at the same time and/or same genre.
I, myself, am open to many types of introduction. However no reader should stumble across spoilers in an introduction. If the writer of the introduction cannot discuss the book without spoilers then they, or the editors, should do the reader the courtesy of marking them plainly and unmistakably.
And yes, I did just have that happen to me. I curled up in a chair with a book I had not read before and glancing at the introduction hoping for insight into the placement of this book in the development of the author's style and choice of topic(s) I came across a massive spoiler. Yes, I will forge on and read the book but (warning to any editor who happens to come across this entry) I have made a note of the author of the introduction, will avoid reading any by the same author and will, if possible, avoid buying editions that include introductions by that author.
Avoiding spoilers and/or clearly marking them is an act of respect to the reader.