Accessibility Services missing
Since the bounty expired (went unawarded) and this question hasn't received an answer yet, I just decided to explain the short term workaround as it may help users facing a similar problem.



Why I needed this feature:


In order to use my productivity apps such as AutoInput, Greenify etc
To be able to monitor which applications whose accessibility settings are enabled at a given moment.
To be able to switch on or off from native interface with ease.






The pre requisites for this workaround include:


Secure settings plugin (in app add-ons)
Tasker ( 7 day free trial with option to purchase)
adb see Minimal ADB setup  and instructions


Workthrough;


First we need to grant Secure settings  these permissions:



    adb shell pm grant com.intangibleobject.securesettings.plugin android.permission.WRITE_SECURE_SETTINGS

    adb shell pm grant com.intangibleobject.securesettings.plugin android.permission.CHANGE_CONFIGURATION





Now we need to open Tasker to create a task for toggling the accessibility service of a certain application.
Go to Task tab and select "New Task + "
You can name this Task anything (e.g Enable AccServ).
Click "+" to add new Action
Code → Plugin → Secure Settings ,[Configuration: {System+ Actions → (select target App to enable(or disable) accessibility service e.g Autoinput:service.ServiceAccesibility}] Tap on "Save"
Tap back to exit from this task
Repeat the proceedure to disable (or enable) accessibility service of the target app (or create a new one).
Exit all the way back to home screen, and long press home screen to activate widgets, swipe and create a shortcut widget which activates (or deactivate the accessibility service of target app).
You should be good to go.




Cons


The major back backdraw is that you are not able to monitor (in real time) the state of the accessibility service of a certain application, you have to rely on switch the toggles back and forth).
It is also generally difficult to control
While this a better workaround than nothing, it is usually time consuming to set up the tasker profiles to handle this.
It clutters the home screen with too many widgets
Relatively battery consuming than using built-in feature in UI.
It requires some paid apps mentioned above




Nevertheless this is a fully functional workaround, which I hope can help users with similar problem