Currently the aria-live attribute for the FormFeedback components is hardcoded to assertive in BFormGroup.

This works fine as long as the state is not changed while the user is typing.
If the state is changed while the user is typing ( e.g. providing live feedback for the current input ) and a feedback element becomes visible, a screenreader will interrupt all current tasks to read out the feedback.
But if the user continues to type, the alert will also be interrupted.
If a user types at a normal speed, he might not even notice the alert through the screenreader.

Having a way to set the aria-live value ( for the case above to polite, so that the screenreader waits until the user has finished typing ) would fix this problem.