Minor: Disabled buttons should give feedback if accidentally pressed
(That disabled buttons do not give feedback.)
Aral: Interestingly, however, for people with screenreaders, the state of the button is read and it does constitute good usability.
Keeping this open so we can have a think about it.
Ideally, you would be able to both disable the button and react if it is pressed. That will have the correct semantics as well as guiding behaviour without violating (albeit my own) “prevent, don’t scold” principle when designing interactions. Some relevant discussion of how this can be achieved technically: https://stackoverflow.com/questions/3100319/event-on-a-disabled-input#comment52017136_3100395