It won't be an arbitrary string on WASI, it'll be a name of the module or the component containing the module. IMO it should return CommandLine.arguments.first on WASI and nil for non-WASI Embedded Swift.
In its current form the proposal making it unavailable on those platforms gets a -1 from me.
Packages that adopted CommandLine.arguments.first work on all those platforms, but if they adopt API from the proposal as currently stated they'll become less portable, which IMO is worse than the status quo.