The Android CDD is a tedious document to pick over, largely because very small changes in wording can have very big consequences. While I would hardly call this one “very big,” it’s still significant: Google has changed the support of standard impedance-based inline headphones controls from a suggestion to a hard requirement. Now, devices with 4-conductor 3.5mm audio jacks must recognize the impedance range and corresponding action it must produce when used on an inline controller.
After years, Google to force Android device OEMs to support [some] standard headphone inline controls was written by the awesome team at Android Police.