Recently, Google has notified developers of apps that use Accessibility features for purposes other than helping users with disabilities to cease using those APIs or otherwise unpublish their app. Luckily, this is Android, so the developers can always just distribute their applications outside of the Play Store through sideloading, but that's not exactly a secure solution for most people - and let's be honest, not being in the Play Store will be the death knell for most developers. The impetus for this move appears to be existence of (now removed) apps in the Play Store which use Accessibility features in conjunction with a vulnerability patched as part of the September security update to install malware. Their latest target is apps misusing Android's accessibility features, which are created to help people with impairments to use Android devices. Many popular apps using accessibility services in innovative ways may be impacted by the warning from Google.
Google's new policy will hurt a large swath of power-user apps. Through the API, an app can see all the other apps the user is running and take an action when a specific app launches.
Accessibility apps can intercept KeyEvents (hardware key presses), which is useful for doing something like remapping the hardware Bixby key on Samsung devices. Of course, this includes older versions of Android that no longer support updates to the Play Store. For instance the "Building Accessibility Services" page suggests developers make apps for users "who may temporarily be unable to fully interact with a device". The email alerts developers that they must explain to app users how their Android apps are using the "android.permission.BIND_ACCESSIBILITY_SERVICE" specifically relating to users with disabilities. Clicking on a button and changing the focus are examples of AccessibilityEvents. The service has also been used for other purposes such as fill in text fields by some apps like Tasker and LastPass to perform some functions.