They rarely document API changes like this in detail in the regular patch notes.
Sometimes we will get a blue post when there are big changes, and the devs in those threads will often stick around and get into the nitty gritty if people have questions, but those are specific to huge changes when they know a bunch of things will break, or when they are introducing entirely new functionality or methodology.
More commonly, they will change or protect an API, a bunch of stuff breaks, a bunch of people will make highly upvoted posts about it, a CM pops up with a “we’re looking into it”, and you get no response for weeks or months, until people either forget about the problem or give up on getting it fixed.
See this 8.2 thread for an example: UI Changes in Rise of Azshara
First post describes a bunch of big changes to how frames are drawn and anchored - huge impact, needs to be documented or a bunch of stuff breaks.
Second post corrects a misstatement from the OP.
Third post is literally “thanks for mentioning this issue, we are looking into it”.
Months later, no response from Blizzard about the sound file replacement they have been “looking into” this entire time. Not in the 8.2 changes thread, not in any of the other threads created on the topic, not on Reddit, nothing.