Thanks so much for all the thoughtful responses, everyone. @yanokwa caught me up on the TSC discussions.
The spec document has been edited to reflect the various feedback received. It's very similar to @Xiphware's strawman above.
You can reach the diff from the last version from File > Version History.
Some things to note:
- asking for user ID and comments have been fully disaggregated as @Grzesiek2010, @Xiphware and others recommended. They are also completely independent from change tracking. This enables various scenarios such as auditing the identity of users navigating the form without including potentially sensitive form data in the log or using reasons for change as "notes to self" in a one-person data collection context.
- the ODK XForms spec is deliberately agnostic to what the user identifier is and how it is obtained.