ICAO Annex 14 requires that runways be equipped, but not that aircraft should not land without guidance.
Operations are in Annex 6 (nothing found).
Annex 14
5.3.5 Visual approach slope indicator systems
5.3.5.1 A visual approach slope indicator system shall be provided to serve the approach to a runway whether or not the runway is served by other visual approach aids or by nonvisual aids, where one or more of the following conditions exist:
a) the runway is used by turbojet or other aeroplanes with similar approach guidance requirements;
b) ...
Don’t forget to check “Contracting States which have notified ICAO of differences”.
P.S. In my day, the ability of a pilot to complete a landing without approach slope guidance used to be a UK CAA requirement.