I really don't understand why you are being so dismissive of many users, just because they don't meet your personal definition of "professional". This change makes some use cases objectively worse, and telling thousands of people to spend hundreds of dollars plus some amount of time to mitigate a change they didn't ask for is not a respectful position, IMO.
I think this change would be fine as a default, but it should be configurable by the end user.
I'm not being dismissive and this has nothing to do with my personal definition of the word "professional". You mis-framing my position doesn't help this discussion at all.
The point is that users never had control of the output on external monitors. This situation is exactly like the situation that's happened multiple times on every OS where someone discovers a way to do something that relies on some function that they either misunderstand or are misusing (think Y2K). Just because it hasn't bitten people in the ass doesn't mean that it's a good way of doing it. People in this thread have commented surprised that Apple hasn't run into this issue during internal uses by its production teams but the reality is that Apple's production teams don't run off the built-in I/O because that's not how you run a video system you need 100% control of.
I agree that Apple should find a way to make this better (or at least hide it on secondary displays) but that's only a workaround. Today it's an orange dot. Tomorrow, it'll just be some other display indicator. The fact is that you can't (and have never been able to) control what displays from the OS on these secondary monitors. The fact that it's just now biting people in the ass is their fault, not Apple's.
One issue is I don’t think software like PowerPoint or Keynote can output to a DeckLink or similar. A quick Google search indicates you need to use ProPresenter, which is a big change for someone who just wants to display PowerPoint slides.
That's partially true but only if you're only looking at DeckLink devices exclusively (which are mostly for full video productions) that rely on some specific things to function. There are other cards that have software that can capture the video from any window and use it as an I/O source. Think OBS but to an I/O device instead of to a virtual camera.
Again, we're talking about "professionals" vs., at best, prosumers and consumers. Those applications are not unusable because of a small dot.