It's bad UX. Put your damned messages where my attention has already been directed to BY YOUR UI.
User notifications on MacOS are definitely standardized, but originally they were Growl notifications until Apple made it a first-party API and iterated on it.
And you are right in that the Growl-style notifications in Mac OS are standard now... but those are different from the ones in question here because they are not related to a control that the user was just manipulating. They could come from anywhere at any time, and thus they must be presented in a location independent of whatever the user is doing.
The Growl-style notifications work well because they're near the top of the screen, too. Users are used to status and information in menu bars and so forth, in accordance with the general top-down convention of presenting information.
Thinking it through, I did actually implement a "toast"-style alert for asynchronous issues in one application. It was at the top of the screen, though. I originally put it in there strictly for debugging, but I think I might have left it in the release. So I'm not entirely opposed to the idea, but mainly its placement in the examples discussed here.
It's pointless pretending there's one perfect guiding philosophy and all others are obviously wrong.
I believe toasts should be confined to this scenario I'm describing, and indeed feedback directly coupled to user focus/input should be located near to that as you say.
Ok, so where does the toast go if you've already scrolled or otherwise navigated to a different area of the UI? These optimistic updated could take multiple seconds to succeed, and maybe as much as 30 seconds to fail.
Not to mention that, if the operation fails, isn't it likely that the user will want to re-try? And that'll require access to the original control in all likelihood.
If the user scrolls away and the information is important, use a modal alert.
These toast notifications just are a bad solution. I often miss them, because I'm, you know, doing work, not scanning my monitors for notifications I mostly don't care about. (Redundancy is not harmless. Redundancy also trains me that your messaging is mostly noise.)
We can have an indicator, then some icon or even a green bar in the "save this" modal, just fine. Or we can make the "archive" icon color different, or add an error, an undo-button, or other message next to it if we really need to convey this information. This could be a tooltip, something in the icon-bar, or anything really: as long as it right at the place where I made the change and expected the change to show up.