We previously discussed platform restrictions when we initially proposed the platforms API, the review thread is here for reference.
I think the main tension we have to solve is between the case where package authors know a certain platform isn't supported (e.g. if the package uses platform specific frameworks such as UIKit) vs. what happens if a new platform is supported by Swift. It is certainly not feasible for the entire package ecosystem to release new versions every time a new platform becomes officially supported and it also makes bringing up a platform harder. On the other hand, it is likely frustrating for developers to depend on a package only to find out that it doesn't build or work correctly on a platform they support.
Ideally, we could rephrase the problem by allowing package authors which frameworks and APIs they need instead of explicitly declaring platform support. For example, if there was the ability to depend on UIKit with a particular version, a package would have automatically be seen as supporting MacCatalyst when it became available. Similarly, if a package only requires Foundation, it would automatically support Windows when that becomes available. This is a pretty big leap from what we have today, though, and likely not feasible in the near term.