A quick setting tile (Android N+) to toggle the state of TalkBack.
$ adb shell pm grant com.ataulm.talkbacktile android.permission.WRITE_SECURE_SETTINGS
This is different from the "pause"/"suspend" setting built into TalkBack because this one actually turns the service off, meaning when you query for the TalkBack status in an app, it's legitimately turned off.
If you don't want to build it yourself (and I would recommend building it yourself, since it requires giving a system permission), you can ✨download the APK from the releases page✨
Thanks Nick Butcher for the reference project, Ian Lake for the article, and Alan Viverette for the hint to look at ServiceControlUtils.