How did chess change over the 2010s and should I keep anything in mind for buying books from before 2010? I presume I am being overly paranoid regarding this, and I definitely don't intend to turn away from older books at all, but were there any developments opening-wise, middlegame-wise, or endgame-wise during the decade that books from before would overlook or fail to cover?
Given that you are even asking this question, I am going to be a bit presumptuous and guess you are, like me, not a highly rated (elo &gt; 2000) player.
For chess books that are not about opening theory, I would not worry too much about the age of the book. Sure, the occasional study or evaluation has been adjusted based on computer engines, but these are surprisingly rare and inconsequential for most of us. Endgame theory has changed little in the last 50 years and that which has is mostly relevant only to very highly rated players. If Bobby Fischer were to teleport from 1972 to today, he would still be a very good chess player and he learned from books which are considered ancient today.
Opening theory does change more frequently but even then, this is mostly of concern to highly rated players looking to extract tiny advantages from deeply memorized computer lines. If your intention is to simply get a feel for a specific opening and its common variations, I think anything reputable from the last 20 years will be helpful, although you should probably complement this by looking at up to date chess databases to make sure popular variations are covered. Which you should do anyway even with a new book.