I was aware of this as a thing that programs often did but wasn't aware that this was a standard. Thanks!
That said, I find that even when programs support this for accessing menus, they often do not support clicking buttons and focusing text boxes using it, so it could be taken further.
They can, and they often do (or used to). It works with both buttons and text boxes – the latter by connecting the text box to an adjacent label.
Have a look at some older piece of UI in Windows and hold down alt for a while. All we need for this to be taken further is for the programmer to pay a bit of attention to the existing feature and make use of it.
For a non-Windows example, I just looked at the XFCE appearance settings dialog. Hold down alt for a few seconds and you'll see that every single control has an access key: all the tabs, all the buttons, all the checkboxes, all the dropdowns.
I'm on Linux and I quickly tried some of my most commonly used software, such as Firefox and Deluge, but they did not seem to be supporting this, unfortunately. Only the menus get their access keys highlighted.
That said, I find that even when programs support this for accessing menus, they often do not support clicking buttons and focusing text boxes using it, so it could be taken further.