I think this is really the right way to go. I'd expect it to be the common case that we care about copyability not just for a specific type but for regions of code or entire modules. I'd argue it's less ergonomic to have to express negative constraints repeatedly for individual arguments or types than to flip the defaults for regions of code (files, modules, or possibly sections of files using something like #pragma); I think it'll be rare to want to think explicitly about copyability for one type or parameter but not for all within a scope.
I think it would make most sense to treat implicit conformance as a module-level attribute that's turned on by default, and that if you passed e.g. --disable-implicit-copyability it would flip the default for that module. You could then re-enable it for sections of code that you haven't migrated or don't intend to migrate using something like @beginScope(ImplicitlyCopyable).
Under this sort of model, you don't need to consider negative constraints; you would write the code in a scope where the Copyable constraint is assumed not to exist.
In this design, all existing modules would be treated such that all types automatically conform to Copyable, all generic constraints have an implicit Copyable constraint, and all protocols descend from Copyable. In a scope that opted out of implicit copyability, those extra requirements would have to be spelled out explicitly, and all types and variables within that scope would need to be explicitly copied (assuming they conform to Copyable) or moved.
With regards to types that we still want to allow implicit copying on: a module built without implicit copyability enabled could introduce its own conformances to a marker protocol ImplicitlyCopyable for Copyable types (e.g. extension Int: ImplicitlyCopyable {} if Int was not marked as ImplicitlyCopyable in its defining module). It could also conform any types it vends to ImplicitlyCopyable, which would also imply a Copyable conformance. Any structs or enums that conform to Copyable would not be allowed to have a deinit.
As a side note: we could apply this same solution to Opt-in Reflection Metadata, since again that's a place where we're wanting to change the default for regions of code – in that case implicit conformance to Reflectable rather than Copyable.