Depends on where you are it seems if at our range in ON steel is hung at an angle and shrouded or not used at all! I use aircraft cable to hang the steel so far so so good.
You are talking about a different target system than the OP suggested and quoting regulations for that different system.
Steel targets
mounted to a stand or similarly restrained are reccomended/required to be at a downward angle of 20 degrees as many manufacturer's specify or suggest the same. Those targets generate bullet fragments with energy because the target is securely mounted and its 'pointed downwards' to control the majority of the bullet fragments. I agree with Slavex in relation to what the target angle predicates, but its a requirement and 20 degrees down makes more sense that any other direction

Further 20 degrees doesn't exceed the critical angle for ricochet and doesn't result in one.
Hanging steel targets, like falling or knock down targets are not required to be at 20 degrees, gongs and hanging plates just hang loose and the bullet dumps its energy into the unsecured target and any fragments just splatter about harmlessly, without any meaningful energy, and if anything continue forward with the target. As such you can hang a gong at its natural 'flat' inclination. Again, none of these targets exceed or meet the critical angle for ricochet and don't result in one.
Perhaps its semantics but I disagree with the term ricochet in relation to the spall or splatter generated by contact with steel targets.
Ricochets suggest an unplanned event of by defintion, and also bring to mind a large or complete bullet that has altered its course, maintained energy and mass, and just sounds downright dangerous and wreckless to the lay person.
In all but the rarest of cases, when a soft item with tremendous energy (bullet) hits a much harder item (AR500 target) the result isn't a ricochet, its the near complete destruction of the bullet. Tiny fragments with very little mass (therefore very little energy) are dispersed in a known and predictable pattern in a place or space that is designed to accomodate them.
Disagree with the terms, call it semantics, whatever you like..but when I'm talking to a lay person or a CFO inspector I don't talk about 'ricochets', as they make the practice of shooting steel sound hazardous to the participants and possible spectators, neighbours, etc. I hear shooters talking about ricochets all the time in reference to Steel targets and I think to myself that isn't a very productive or purposeful description if you want to keep your range open and not bear more scrutiny than you are already suffering.