The certification regulations might assume a maximum of 3.5 deg, but I cannot find a reference. Also 3.5 deg may the margin which a manufacturer has to show as being safe via development simulation, whereas the AFM value probably matches that demonstrated by flight tests.
An ‘ultimate’ limit depends on the aircraft type – aerodynamics, the flight guidance control laws, and the means of establishing a GS beam. RAE flight tests demonstrated 6 deg autoland with a BAC 111 and 5.5 deg with a HS748, flying MLS glidepaths.