I think that toasts are kind of an attractive nuisance when it comes to accessibility.

They can technically, with ample constraints and a great deal of restraint, maybe end up complying with WCAG, etc., but all it takes is one developer saying "well a toast is easy" or "this isn't that important, make it auto-dismiss" and you're back in bad pattern town.

You see this with government web design systems - they have a very limited and constrained palette of patterns, because it allows for more consistency and reliable accessibility, versus having a bunch of tools that you just generally shouldn't use.

(The GitHub page linked above also makes a great case for how "making toasts accessible" isn't as simple as just having the right aria roles - lots of details the Adobe design doesn't seem to completely cover, unfortunately)