Since the question is phrased in the abstract and not with regard to one model, here is the abstract answer: While in general calipers offer a choice of pad compounds, calipers do differ in how well they manage heat, and there are various expense- and weight-adding design elements some calipers have that help them manage it much better than others. Metal pads in particular tend to go along with riding styles that involve sustained heavy braking. The kind of riding where the pros of metal pads outweigh the cons is also the kind of riding where hypothetically a manufacturer could see fit to say that recreational-level caliper model X is outside of its intended usage, and will be overly prone to fade, need frequent bleeding, and so on. In practice, manufacturers issue intended use guidelines for their brake models based on riding type and loads carried. I haven't seen any explicitly do it by saying a certain pad compound is incompatible.
Shimano likes to stick to a tech support and liability management strategy of saying "this caliper takes this model pad and can be used with these three rotors" and the like, even though there are other pads and rotors in their lineup that may be physically compatible in terms of form factor. For example:
There are many other rotors, pads, and levers in the lineup that are physically compatible in practice, but these are the ones that are warranted and that they stand behind. It is easy as a consumer to criticize this practice and see cynical aspects to it, and it can be frustrating at times to deal with the world of bike parts compatibility as presented by Shimano because it often leads to a need to read between the lines just to make things work or do the right thing, especially when working with old parts, but one must also understand they've chosen to be in the business of selling bike parts systems that are designed to work together. That's simply their prerogative, and note also that while it may result in a lot of nuisances for people dealing with their parts out in the real world, they make almost all their money selling to bike brands who are speccing out bikes and need systems of parts that will just work, which is part of where the "X works with Y and Z and that's it" ethos comes from. Yes they have to counterbalance that against the risk of reputational damage from their prissiness about these issues, but it's self-evident that the balance they strike works for them.
In this case, Shimano doesn't scream out loud in their own literature that BR-MT200 must only be used with resin, but the rotors they spec for it are resin-only (cheap, not as hard) and the pads they spec for it are resin. They're trying to guide people away from pushing the brake beyond what it's really made for.
BR-MT200 is a good example of a brake that hotrods pretty well but was still only developed and marketed as an entry-level, recreational product. If it does better than they may have expected under demanding conditions and people are pushing it hard with some success, that's not necessarily Shimano's "fault" and it still doesn't oblige them to recommend it as anything more than an entry level product. Some manufacturers have taken to speccing it on cargo ebikes, which I think is unwise and pushing everyone's luck too much, even though it mostly works out okay. In my opinion one of the things we're all just going to have to see about is how well they and other plastic-piston brakes hold up over time on such bikes, but the result will be interesting either way.