Accessibility is like all other things in that, "You can't please all of the people all of the time," and there is the added bonus of trying to decide what information to expose to a screen reader. I have tried to explain, from a sighted perspective, just how much of what appears on any screen that those of us who see do ignore - we just filter it out as non-significant background clutter - except on the very rare occasion we might have an occasion to use it. And a great deal of this kind of stuff is truly never attended to visually or otherwise by a majority of users. Some of it is there as much for troubleshooting purposes as anything else.
Joseph Lee first introduced me to the concept of "information blackout" for screen reader users, and because I had already tutored many in learning basic skills the concept immediately clicked. As someone with sight, I can take in the entire gestalt of the screen and, without even realizing I'm doing it, processing information so that I know what to attend to and what to ignore. A screen reader user can never do this and, even worse, is largely at the mercy of how well something is coded (particularly web pages) in terms of what the screen reader itself encounters first and presents to them. If you rely on just sitting there and letting the screen reader read for anything other than completely unfamiliar material, you end up wasting a huge amount of time, and sometimes do for even unfamiliar material. Using things like the screen reader search function, when you are virtually certain that the thing you are looking for is present and you know what word(s) would identify it, is what lets you zero in on the actual content you want. That and using things like the NVDA elements lists to "hit the high points" in exploring material quickly rather than hoping you'll trip over something via reading. But even if you are the perfect, ultra efficient screen reader user, you will still never be 100% certain that you've seen/heard everything, and that's even if you allow a read all from the start. You really can't take it all in at once and do a preliminary filtering, and in any visual medium (and all print media like web pages, magazines, etc.) are visual media.
The Thunderbird status line has always been visible by default, but it clearly has not had its content exposed to screen readers in the manner it is being exposed now. I understood instantly what the problem was when I was watching and listening to someone using beta Thunderbird, and why it would drive one to madness in short order. I can't imagine how few instances there would be in daily life where a screen reader user would give a fig about information being presented by the status bar when they are in the midst of actually reading e-mail. It's an absolute intrusion, and one that doesn't allow you to absorb the content of your message nor the status bar in any useful way as well.
Someone made a boo boo. Who knows who, or why, but at least it can be remedied if identified. And even though turning off the status bar works, since it's on by default and you probably don't care about it then it's worth telling the Thunderbird folks what a hash this change has made.
Brian - Windows 10 Pro, 64-Bit, Version 1909, Build 18363
Power is being told you're not loved and not being destroyed by it.