The note regarding notching the top flange for access if plate extensions are not acceptable is a way to limit any potential trip hazard. If the base lands on a raised plinth then this should not be a problem. If it lands on a flat slab then access around the base should be considered and the top notch approach should be used to limit any trip hazard. Alternatively move the plate back below the bottom tread and have the bolts inside if possible. This will mean fixing the bottom tread after the base has been fixed so not as suitable if the stairs are pre-assembled. Something to consider if a trip hazard is likely.