Does the yeast weaken in flat beer? Does the yeast in beer weaken when the beer is flat? I use the beer in making bread, and, since I don't drink, it sits in the fridge until I bake bread again.  I'm curious whether the yeast weakens in flat beer.
By the time you open your beer the yeast has mostly exhausted itself by converting the sugars into alcohol.
You may be able to use it to kick start a sourdough starter, or grow it back to an active bacteria, but it will not be able to leaven a batch of bread by itself from the bottle.