The maximum ILS glide-slope angle is probably defined by an aircraft’s aerodynamic capability – e.g. shuttle; for commercial aircraft a steep approach certification.
The original LCY glideslope was 7.5 deg (for noise reasons), and because the only aircraft planned to use the airport was the Dash 7. This aircraft was certificated in the STOL category.
LCY was reduced to 5.5 deg to accommodate the limit of BAe146 / Avro RJ steep approach capabilities (although some 146/RJ’s have a 6 deg clearance)
With increasing angle at constant beam-width, the apparent GS sensitivity decreases (greater altitude deviation for the same GS indication), thus the beam-width may have to be narrowed. If so, then as far as the crew is concerned, they continue to fly within the normal limits to provide adequate obstacle clearance. However, crews should be aware that they may less time to response to a deviation due to the higher descent rate, i.e. with a GS deviation you have less time to recover back to the beam centre. Similar adjustments may be made to LOC instalations on short runways.
IIRC, planning for LCY jet operations considered the use of a Cat 2 quality glideslope installation (beam accuracy, stability, etc) so that a smaller obstacle clearance margin could be claimed to ease some for the problems of close in obstacles.
Also, with failing memory, Plymouth (UK) had a test / trial ILS installation at 7.5 deg, as a switchable alternative to the normal 3 deg beam.