That was what I was thinking. But I can see your point. I don't feel strongly about it one way the other.
That's a good point, @Grzesiek2010. I've never actually seen these in use so I forgot about them. But indeed, https://opendatakit.github.io/xforms-spec/#metadata does describe simSerial and subscriberID metadata properties. I propose that we deprecate these in the ODK XForms spec. @martijnr, has Enketo been able to access these?
I'll amend my original post to add this.