The advertised FOV is before any rectification occurs. During rectification, both the left and right images are “stretched” in order to create a best fit, rectangular crop of the combined two if you were to place them both on the same horizontal axis (this is the image that you would see during stream). Therefore, you will lose a portion of the FOV, particularly, portions in the corners of the “stretched” images (diagonal direction outward from the center of the image). Exactly how much you lose is unique to every unit due to a potential (small) vertical offset of the left and right images (stemming from mechanical imperfections in machined parts).
The exact FOV after rectification cannot be given a standard value. The FOV after rectification will always be less than or equal to the FOV prior to rectification.
Furthermore, the longer the focal length (smaller unrectified FOV), the less FOV you will “lose” after rectification as a percentage.