Re: with the new option turned off, (account list is active), the user will not be able to get out of the app, only by pressing on Back button, from message list view
Yes this is true.
We removed "tap back to exit from message list" and replaced it with "hide account list view".
Yes there is one case that's not possible anymore but I don't think it's common. There are probably others, likely even less common.
Keeping every possible option and their combination as we move forward isn't practical - it would require more and more time and effort and result in more and more subtle bugs. Some things have to be kept, others are best removed / replaced.