The last message on this bug report was over seven years ago, and in that time I think a solution to this problem has come up that won't require adding a new field to debian/control. Perhaps if an Architecture: all package needs to be built on some architecture in particular, then that package could specify Build-Depends-Indep: native-architecture-is-amd64. I believe this should work today; native-architecture-is-amd64 is a virtual package provided by native-architecture-is:amd64, itself built by src:architecture-properties.
This generalization also seems more useful and semantic; hypothetically, if a package requires more memory to build than can be provided in a 32-bit address space, this could be hinted with something like "Build-Depends-Indep: architecture-properties" and "Build-Conflicts-Indep: architecture-is-32-bit". So it seems to me like this can already satisfy any current or similar use cases, and perhaps no further action is required.
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